I can't improve on the excellent advice that the foregoing posters have given you, but I will add a trifle. First, a little bit of nomenclature: that last bit of tobacco is called the dottle, and some smoke it and others don't. One thing to keep in mind is that the dottle acts like a filter. How so? Because every puff you take passes through the tobacco remaining in the bowl, until it becomes what we call dottle. Now, as you might imagine, the dottle is going to be a bit moister, but it's also going to have a lot more tar and nicotine than it did when it started out a short while earlier as dry tobacco in your pouch. For some stout-hearted fellows, this is no problem--they love it, and smoke it all the way down--depending on the type of tobacco. As previously mentioned, it's just about impossible to do this with an aromatic, though some manage it. If I have a pure virginia, I can do it. Add Perique to the mix and I can't. English tobaccos--well, it depends.
You see, there's no need to feel bad about not smoking dottle. If you've been smoking aromatics, chances are the dottle is way too moist to take a light and keep it going. Also, lots of people let their tobacco sit out for a while to dry before they smoke it. As Jason said, "think bone dry." If it crumbles to dust it's too dry, but ideally it shouldn't be too far from that point. If, by the time I get down to the dottle I find the taste getting a bit strong or less than pleasant, or I feel too much Vitamin N coming through, I set the pipe aside. Other times I'm able to smoke down to a clean white or grey ash. It really depends on which pipe and which tobacco. But here's the thing: if I smoke it all the way down to the last puff, I don't feel a sense of triumph. Similarly, if I reach a point where I decide that more wouldn't be fun, I don't feel a sense of failure. Pipe-smoking is about your pleasure--nothing else. If you're enjoying the bowl up to the dottle, that's great--you've won! You might try drying the tobacco out a bit to see if that helps, but some tobaccos are just about impossible to dry out, because they contain humectants, which means they draw moisture right out of the air, so that's a fight you're not going to win.
Here's the deal--if you can smoke 3/4 of the pipe and feel that you enjoyed the experience--that's all you need. Trust me on this one--I've been smoking a pipe for just a few weeks shy of 55 years, and sometimes I go all the way to the bottom, and sometimes I don't--and I consider myself a reasonably experienced smoker. Sometimes I know--or think I know why the pipe went out on me, and let me tell you--there have been plenty of times when I smoked it all the way down and wished I hadn't. Let your taste buds guide you.
OK--that's a small amount of information crammed into a lot of space, but it's good to know stuff, and it piggybacks onto what the others have told you. Remember Rule 1: Enjoy your smoke!
I smoke Nightcap from top to bottom on single light just by tamping good every now and then. When you feel the draw becoming too loose - tamp.
