The issue is, why can't you get the drums to sound as big as the drums on big, commercial releases. In my opinion, with budget gear, this is not really possible. I've struggled with the same thing. The fact is, there are so many things that happen to the sounds between sampling them and getting them to the cd. First, they go through A/D conversion to your mpc. Fair enough, even the pros work like this. But out of the mpc, big producers will run sounds through high end compressors, mic pre's, juicy eq's, etc, and then out to probably an SSL console. Wonder how the first WU album's drums sound so gritty and still bang? That's raw, low bit rate samples run through an SSL (or something similar). The theory that because you take samples from great sounding records will make your tracks sound just as good as the original record does not hold up. Hell, everyone has access to great sounds on records, but when you listen to the music people post on this website, does it sound like a big record? No. (not trying to take away from anyone's work, just reality).
But this doesn't mean you can't make banging drums. All the techniques posted here are good. Also, for the drum layering thing...I've found that this is the best way to improve your drums. A lot of times I wish I could make a single snare sound better, but by adding another snare or clap you can get something good. Second, you give yourself a better chance if you have a good signal going in to the mpc. This means a good cartridge, needle, and I would even say that a few hundred dollars spent on a good phono preamp will help a lot. Get the drums as big as possible going in, this will help offset the fact that you don't have big time gear do this going out of the mpc. And - I never used to think cables made a difference - but if you have any connection over ten feet, and you have real crappy radio shack cables, and you replace them with good monster cables or something, you will definitely notice the difference. All these things cost money but will give you the most bang for the buck.