When I'm using my hand saw to make various cuts, I find it extremely difficult to make a straight cut. Where am I going wrong, what I am missing that the pros seem to have down?
One very important step in using a hand saw is not to force the saw. You shouldn't be trying to force the saw through the wood. The cutting blades are sharp. Once you have a groove started, ease up on the saw and let the teeth do the work for you. Sure, it takes a bit longer at first, but you are rewarded with straighter cuts with cleaner ends.
Secondly, when you're using a hand saw, the power from the stroke should be coming from your body stance. If you're right handed, stand with your left foot further out than your right and lean with your left side into the direction of the cut. Hold the saw such that your index finger is pointing along the saw in the direction you want to cut. This will help keep things aligned.
One obvious solution is not letting the saw do anything that you don't want it to do by using a guide. In the easiest case, the fingers of your left hand are the guide, much like a professional chef would handle a knife, except one normally uses the thumb's nail for that with a saw.
Alternatively, you can simply fasten a straight piece of wood with a screw clamp. Any old block of building wood with an straight, orthogonal cut at the end will do.
Preferrably, your guide should have a small bevel (really small) on the bottom edge to account for the fact that most saws have set teeth.
You can of course also buy guides which will hold your saw in place with rare earth magnets, too (optionally even with a particular angle). Those are much easier to handle and more fancy, but also a bit more expensive than a block of building wood. I use these for cutting minute dovetails sometimes. They do give a better surface than sawing free-handed.
Then of course, using the correct saw makes a huge difference as well. There exist saws for rip cuts (along the grain) and for cross cuts (orthogonal to the grain), and "universal" saws which work equally
bad well for either direction.
Also, there exist "western" and "eastern" (japanese) saws, the former cutting on push and requiring condiderably more force, the latter cutting on "pull" and usually being much slimmer (0.2-0.6mm) and having much smaller teeth.
Which saw is the right one for you is a matter of taste, but I very much prefer japanese saws (dozuki whenever possible, kataba if you need to go deeper than the back will allow, ryoba for the more coarse stuff. Ryoba is typically available with rip on one side and cross on the other, which is very nice).
Pulling with two fingers is a lot more comfortable (and I'm inclined to believe much more precise) than working with force. But that's a bit of a matter of taste, too, you'll have to try.
Here is a photo of a guide that costs about 4€ in material (of which 1€ is for the neodym magnets in the little holes) and 5 minutes to manufacture:
Thanks to the magnets, the saw will cling to the guide nicely. You get 100% straight, perpendicular cuts every time. Similar guides are available commercially from Veritas (for about 10-12 times the price).
I am a beginner myself with hand sawing, but the following tips worked for me for straighter saw cuts:
Mark your cuts either with a pencil or marking knife or both. This allows you to see if you follow the line. The added benefit of using a marking knife is that it severes wood fibres, so there is less tearing or splintering on the exit side.
Do not force the saw, just let it do its job. For Western-style saws, forward pressure is OK, but do not apply (a lot of) downward pressure. Relax your grip on the handle -- it may help if you imagine holding a tiny bird in your hand that you are trying not to crush.
As others have pointed out, your elbow should swing free of your body.
How you start the cut can make a huge difference. For particularly delicate cuts, I cut a straight V-groove with a marking knife or chisel that helps the blade to start right at the planned angle. Again, no forcing the saw and no downward pressure at all. I make a few pull strokes to get the blade catch in the wood (pros I've seen on YouTube start the cut with short back-and-forth movement -- didn't work for me, I guess it depends on the wood).
Another handy trick when starting a cut is to watch the reflection of the wood on the saw blade to see if you are holding it perpendicular to the surface.
Once the cut is started, control the saw by focusing on where the saw should go, as opposed to where it is. I know it sounds vague, but for some reason it works.
Use the full length of the blade. In my admittedly limited experience, this results in straighter, cleaner and faster cuts + the wear on the blade is more even. It helps if you imagine the saw is longer than is actually is.
If the saw starts to wander, do not bend the blade. Instead, track back to where the saw started wandering and try again.
Finally, practice a lot: a single scrap piece of wood offers plenty of opportunity. For what it's worth, I stopped beating myself over cuts that wandered off 1-3 degrees and bought a low-angle block plane, built a simple shooting board and now I can straighten a slightly skewed cut.
One time honored method for achieving square cuts is to use a miter box. Check out this article on Ditch the Miter Box for an interesting discussion of pros and cons. The main article suggests abandoning miter boxes and using a back saw without the box for making precise cuts.
No matter how you make the cut, one sure aid in getting a straight cut is to first draw the line along which you wish to cut, then place the saw on one side of the line. Never try to saw down the middle of the line because you are essentially erasing as you go. Even if you start by putting down a saw kerf, make it on the side of the line that is going to give you the desired length of the piece of wood.
I'm a week late to this question but I'll throw in a bit of my experience.
Hand sawing does involve much body mechanics as was previously described. Assuming your shoulders are aligned and you're moving your arm as you should be, the only other thing that might make the saw wander off the cut line is misalignment of the teeth. Otherwise known as the "set" of the teeth. This is never a problem with new saws but is almost always an issue with something found at a flea market.
I sent on of my saw off to Matt Cianci once who sharpened and set the teeth and did a masterful job of it. It was astounding how well the saw cut. Since then, I have attempted it myself with another old saw with mixed success. The last attempt was successful at getting the teeth all set properly.
My point is that once the teeth are set properly and you have your arm swinging in line with the cut, the saw should track straight.
My favorite videos for learning this and other woodworking techniques are those in the YouTube channel of Paul Sellers. He has a gift for explanation and demonstration. He also has a paid video service which I have not tried yet.
I try to See what the saw is doing with each motion. I used to just saw away and not really see what was going on.
The main problem I see is people tilting the saw in the z-plane as they cut. Keeping the saw blade straight up-and-down or orthogonal to the piece being cut has helped me cut straighter.