It sounds like your drill has Nickel Cadmium (also called NiCd or NiCad) batteries. This type of battery self-discharges very rapidly and will not hold a charge as long as a Nickel-Metal Hydride (NiMH) or Lithium-based battery (such as Lithium-ion or Lithium-polymer).
Matt's answer includes a lot of technical detail on how to best store and maintain your batteries. I won't repeat that information but I will mention a few very noteworthy points regarding each of the three battery technologies commonly used for cordless tools.
NiCd is basically worthless when it comes to long-term storage:
When not under load or charge, a Ni–Cd battery will self-discharge
approximately 10% per month at 20°C, ranging up to 20% per month at
higher temperatures. It is possible to perform a trickle charge at
current levels just high enough to offset this discharge rate; to keep
a battery fully charged. However, if the battery is going to be stored
unused for a long period of time, it should be discharged down to at
most 40% of capacity (some manufacturers recommend fully discharging
and even short-circuiting once fully discharged), and stored in a
cool, dry environment.
You'll get more recharge cycles out of Lithium-ion (Li-ion) batteries if you recharge them once they drop to 20% capacity, and as Matt mentioned, you should store them at 30-40% charge.
NiMH batteries for tools are not very common in the US. Older NiMH technology had an internal discharge rate of about 3% per month, which is better than NiCd but not as good as Li-ion. Newer NiMH batteries have a very low internal discharge rate and can retain a charge better during long-term storage.
What does this mean in practical, real-world terms?
All this talk about storing batteries at 40% charge or topping off Li-ion batteries at 20% charge is almost purely academic for various reasons. You can have the best intentions of following the guidelines, but it's a pain in the neck to try to adapt your own behavior to suit the battery technology. The best advice I can give is to just buy a battery technology that doesn't require a babysitter. In the US, that's probably Li-ion. Outside the US, it may be Li-ion or NiMH.
As you said in your question, "there are times when life gets in the way...." Unless you're able to dedicate significant time to woodworking on an ongoing basis, you may not know when you'll use a tool again so it's impossible to know whether you should prep your batteries for long-term storage.
Some batteries or tools do have integrated meters which show you the battery's current charge level. One of my Li-ion drills has a 3-bar indicator but it isn't particularly accurate--by the time the tool reaches 1 bar, the battery is practically dead. If your tools or batteries don't have accurate charge meters or don't have any meters at all, good luck figuring out if you're charging your batteries at the right time or storing your batteries at the optimal storage capacity.
I've owned drills and other types of devices with both types of batteries that are common in the US, NiCd and Li-ion.
Every time I've gone to use my NiCd drill, it has been dead or it didn't last long enough to finish the job. I basically had to charge it every time I wanted to use it, and that takes several hours. Usually I would end up with an unplanned window of time during which I want to get something done, and that window of availability was used up just waiting for the battery to charge. When I did get to use the drill, I sometimes would deplete one battery and put it on the charger while I switched to the other battery and continued working. By the time I drained the second battery, the first one was still barely charged. Granted, there are more advanced NiCd batteries and chargers that allow for faster charging, but they are not always available for a given line of cordless tools. If they are available, they often aren't included and have to be purchased separately--in which case, you perhaps could have just chosen a more hassle-free type of battery.
In contrast, when I go to use my Li-ion drill, the battery loaded in it is always ready to go and has at least some charge, and the second battery is ready to go as soon as the first one runs out of juice. The batteries only take about 30 minutes to charge and last a long time. As soon as the first one is drained, I can put it on the charger and I know it'll be fully-charged by the time the second one runs out of juice. I've had the drill (and batteries) for several years now and haven't noticed any degradation of the batteries.
Summary
In just a few words, NiCd batteries require a lot of babysitting, NiMH batteries are better, and Li-ion are the lowest-maintenance.
No batteries do very well in extreme heat or extreme cold. During the summer, I store my batteries in my garage shop and try not to leave them in my car if I need to take them elsewhere. If your shop gets extremely hot, you may want to store your batteries someplace cooler. During the winter, I store my batteries inside my house and don't leave them in the garage or in my car to prevent them from freezing.
If you use your tools every day or even every week, NiCd might work for you. If you regularly go more than a month without using your tools, it's probably safe to say you sometimes go several months without using your tools, and you'd often need to charge your NiCd batteries before you can use them. In that case, you're probably best served by Li-ion or NiMH batteries.
Fortunately, many lines of tools now offer compatible batteries in both NiCd and Li-ion types. If you already have a tool with NiCd batteries but NiCd is not suitable for your work patterns, you may be able to "upgrade" to a Li-ion battery by buying a compatible replacement Li-ion battery (and Li-ion charger), or by buying a new tool whose batteries are compatible with your existing tool.
