I'm still somewhat of a beginner at home recording, but I'm a huge SP fan and I've recorded some Pumpkiny stuff... so maybe I can help.
Just the basics-- it helps to pan the tracks so that they're spread out around the listener. Also, you should be using a drummer or at least a drum machine, if you're trying for that full band Pumpkiny sorta sound... and bass of course (preferably with an attractive female bassist). And I definitely agree with what Hard2Hear said about how timing is crucial. If you can record the part exactly the same all 15 times, then you'll be okay. DON'T think that just because there are a lot of tracks that each one doesn't have to be perfect. They do have to be perfect! Or it will turn into a mess.
If there's one useful thing I can tell you about all this, it's that you should check out some live Pumpkins material (they just rereleased a live CD/DVD from '94 called Earphoria and Vieuphoria, respectively). You'll see that they did a pretty good job of getting that thick layered sound live, even with just two guitars. So it's not always studio magic... a lot of it has to do with the guitars, effects, and amps. Personally, I've tried the layered guitar approach, and now I'm finding that I prefer to just have two rhythm guitars.