Return to Answer

###The just-over-half trick When chiselling out, do not start at the beginning of the to-be-removed piece. Instead, start somewhere in the middle, and go down in an angle. Once you are certain that you are done just over half, stop working. Do the remaining pieces on the same side, and always stop once you are just over half done.
Now turn the board around and start from the other side. This guarantees that both sides have a clean, perfect edge, and no visible tearout is possible, no matter what. If you are a bit in a hurry, you can give it a hard hit with the chisel once you're 1/3 done from the second side, that will do away with the to-be-removed piece.

###The clamp trick Clamps can be used not only to press the joint together in the direction of the two boards. A third clamp can be used to slightly compress the dovetails. It will only be maybe half a millimeter over the whole board, but that is enough to hide the visible gaps of a normal manufacturing precision and turn a "meh" joint into a "woah" joint. The glue will hold everything in shape once it is cured.

###The water trick This is a trick that a master carpenter told me (I haven't tried it myself, but once you think about it, it's so darn obvious that it simply must work). Wood expands when exposed to moisture. So, if you have visible gaps, give it a mild stroke or two with the sponge, and wait a few minutes.
Obviously, having a glue that isn't too sensitive to water (or one that even prefers the presence of water, like PU glue) is necessary here.

###The dirty trick If, after all, you still have "meh" quality joint with visible gaps, fill the gaps with a mixture of sawdust and glue. The sawdust will give the glue a convincing color, which will make the gap look a lot less prominent.
For that reason, it's always a good idea to preserve some of the sawdust or wood shavings.

The just-over-half trick

When chiselling out, do not start at the beginning of the to-be-removed piece. Instead, start somewhere in the middle, and go down in an angle. Once you are certain that you are done just over half, stop working. Do the remaining pieces on the same side, and always stop once you are just over half done.
Now turn the board around and start from the other side. This guarantees that both sides have a clean, perfect edge, and no visible tearout is possible, no matter what. If you are a bit in a hurry, you can give it a hard hit with the chisel once you're 1/3 done from the second side, that will do away with the to-be-removed piece.

The clamp trick

Clamps can be used not only to press the joint together in the direction of the two boards. A third clamp can be used to slightly compress the dovetails. It will only be maybe half a millimeter over the whole board, but that is enough to hide the visible gaps of a normal manufacturing precision and turn a "meh" joint into a "woah" joint. The glue will hold everything in shape once it is cured.

The water trick

This is a trick that a master carpenter told me (I haven't tried it myself, but once you think about it, it's so darn obvious that it simply must work). Wood expands when exposed to moisture. So, if you have visible gaps, give it a mild stroke or two with the sponge, and wait a few minutes.
Obviously, having a glue that isn't too sensitive to water (or one that even prefers the presence of water, like PU glue) is necessary here.

The dirty trick

If, after all, you still have "meh" quality joint with visible gaps, fill the gaps with a mixture of sawdust and glue. The sawdust will give the glue a convincing color, which will make the gap look a lot less prominent.
For that reason, it's always a good idea to preserve some of the sawdust or wood shavings.

Mark the desired shape of your tails/pins. You are in principle free to choose any angle, and any regular or irregular layout or proportions that you want, there are very few "rules" that are kind of you should follow. There are entire religions about which is the correct formula for the width of tails, but I've done them in many variations including irregular widths, and you can't really tell that one or the other is truly "wrong".
As a rule of thumb, try to get a somewhat even distribution for the beginning and do the outermost pins half-width (looks pleasing), and go for somewhat steeper (1:6 or even 1:4) angles if you use soft wood or very thin boards, and rather not-so-steep angles on hardwood. You can use a selfmade jig or a commercial one to get the correct angle, or simply measure. All that is just a rule of thumb, you can in principle do whatever you want.
Although at this point, you do not yet need to be super precise on marking, try to be precise anyway, use a sharp pencil or a kogatana. It never hurts to work with diligence, and you should get the habit of doing precise marks, so do it anyway.

All accessible lines (that is, any lines that has contact with an outer surface) are cut with the saw, then the rest is worked with the chisel. First mark the line with one exact hit, cutting fibers. Then remove the cut-off chip. Begin exactly orthogonal to the board by using a small piece of wood as a guide. After the first millimeter, you are allowed -- and encouraged -- to "cut behind" a little (I am not sure about the correct term to use in English, the German word is "hinterstechen"). Doing so will make it a great deal easier fitting pieces together later while preserving a perfect visible edge on the outside.

One you have finished one side, hold it against the other side, and carefully, diligently, mark its contour onto the other piece with your sharp pencil. Here, it really counts, so take your time, be precise.

Mark the desired shape of your tails/pins. You are in principle free to choose any angle, and any regular or irregular layout or proportions that you want, there are very few "rules" that are the kind you should follow. There are entire religions about which is the correct formula for the width of tails, but I've done them in many variations including irregular widths, and you can't really tell that one or the other is truly "wrong".
As a rule of thumb, try to get a somewhat even distribution for the beginning and do the outermost pins half-width (looks pleasing), and go for somewhat steeper (1:6 or even 1:4) angles if you use soft wood or very thin boards, and rather not-so-steep angles on hardwood. You can use a selfmade jig or a commercial one to get the correct angle, or simply measure. All that is just a rule of thumb, you can in principle do whatever you want.
Although at this point, you do not yet need to be super precise on marking, try to be precise anyway, use a sharp pencil or a kogatana. It never hurts to work with diligence, and you should get the habit of doing precise marks, so do it anyway.

All accessible lines (that is, any lines that have contact with an outer surface) are cut with the saw, then the rest is worked with the chisel. First mark the line with one exact hit, cutting fibers. Then remove the cut-off chip. Begin exactly orthogonal to the board by using a small piece of wood as a guide. After the first millimeter, you are allowed -- and encouraged -- to "cut behind" a little (I am not sure about the correct term to use in English, the German word is "hinterstechen"). Doing so will make it a great deal easier fitting pieces together later while preserving a perfect visible edge on the outside.

Once you have finished one side, hold it against the other side, and carefully, diligently, mark its contour onto the other piece with your sharp pencil. Here, it really counts, so take your time, be precise.

Use a marking gauge to mark the length of your pins/tails so you have the same length everywhere. Copy the required thickness from the board, do not measure. Some gauges have a micrometer screw which lets you adjust the setting. Those are normally not very useful, but here they can be really handy: You can optionally make the tails/pins a tiny bit longer if you want by giving the micrometer screw one or two turns.
The rationale behind that is that if your tails/pins are a tiny amount too short, the joint doesn't fit properly, it looks eally bad. Which sucks because there is nothing you can do about it except spend 4 hours on the sander thinning the whole board down. On the other hand, if they are a tiny amount too long, it takes two strokes with the smoothing plane, or 15 seconds with sand paper, and it looks absolutely perfect. Preferrably, of course, tails/pins should be exactly as long as they should be, but very slightly too long is harmless whereas slightly too short is a desaster.

Use a marking gauge to mark the length of your pins/tails so you have the same length everywhere. Copy the required thickness from the board, do not measure. Some gauges have a micrometer screw that lets you adjust the setting. Those are normally not very useful, but here they can be really handy: You can optionally make the tails/pins a tiny bit longer if you want by giving the micrometer screw one or two turns.
The rationale behind that is that if your tails/pins are a tiny amount too short, the joint doesn't fit properly, it looks really bad. Which sucks because there is nothing you can do about it except spend 4 hours on the sander thinning the whole board down. On the other hand, if they are a tiny amount too long, it takes two strokes with the smoothing plane, or 15 seconds with sand paper, and it looks absolutely perfect. Preferably, of course, tails/pins should be exactly as long as they should be, but very slightly too long is harmless whereas slightly too short is a disaster.