Im pretty sure everyone covered all the techniques for getting a big fat kick... but you could also go the other way.
If you listen to Pendulum (drum and bass band, totally dope), they have an interesting tactic I employ all the time.
Get a nice punchy kick with a lot of mid/high end. Now, load an EQ and completely cut off the sub bass area. Then, load another EQ and boost it so that the kick is very clicky. Go to your basslines and cut all the areas you boosted your kick so it punches through (Im a big fan of subtractive EQ), and then with everything else in the track, cut the sub bass range completely off. Then make a sine wave sub bass, make sure it covers the entire sub bass frequency (usually takes two octaves) and then cut everything in its EQ except the sub bass freqs, so there isnt any overlap. This way you can get basslines with lots and lots of definition in the bass-mid bass area, get a nice punchy kick and have car-blasting sub basses.
Make sure you cutoff any super super deep freqs on your sub bass to save headroom (Wherever you cutoff when you pre-master or mix it is good. Some people say 20 htz, some people say 30. It depends on your habit.)
If you take a track by Pendulum and lowpass it so you can only hear the sub bass, it has zero kick interference and lots of pure sine basses. And really, they have FAT basslines.
Yeah, this didnt answer your question, but its kind of an alternative. Its also geared toward more classic drum and bass, rather than dubstep, so this may or might not be a good idea.
Now to make your kicks big and fat... Well, first do everything else everyone else said (layer two well EQed kicks, saturate them, compress them), but then do an EQ sweep and find the real sweet spots, where the big 808 bass hits and where the real punch is, and possibly where it clicks in the high end. Boost these frequencies (might be a good idea to do this before the compression). Now, you have a big punchy fat kick right? Now here is the thing you need to do: make it cut through the mix. Take your basslines and violently subtract these frequencies from them with an EQ so that the kick really stands out in those areas (dont cut them completely, or your bass will sound thin, but do them liberally). Then take your percussion, like your snares, toms, hihats. EQ them all so they have no bass frequencies that are interfering with your kick and your basslines. Do the same with all your instruments, so that the kick has a real place in the mix.
Now you have your kick that stands out in the mix. If it is overpowering your bassline now, try bringing down the frequencies where your bassline is really hitting.
I am a huge fan of subtractive EQing... I find that when I take away frequencies, I can stuff way more power into a mix. You may have to layer more tracks though to make everything have a bigger frequency range before you cut, just to make sure that they arent thin. You want them big and bold in their spectrum, not thin and wimpy
. Currently I'm using Fruity Loops 10!
