These are great tips.
Sometimes, depending on what type of kick you have, there's a trick to add the harmonics to the kick, and it fattens it up a bit. Most hip hop heads just use a low-pass filter and take everything out from, say 150 Hz and up (or 200, 250, depending on the kick and the low-end they're trying to achieve. But I've done this once or twice, and it's the opposite, but it fattens it up a little:
On a [real] kick drum sample: Say you find that the heart of a particular kick is 75 Hz, and that's the center frequency that it "sits" at. Take 150 Hz, 300 Hz, and 600 Hz, (even 1.2 kHz) and bump them slightly, each higher frequency bumping less than the previous. Also, altering the bandwidth (Q) so that the space in between is attenuated, so your spectral EQ actually resembles a waveform.
This is just a weird tip that can make a thicker kick.
Other than that, the advice on the above is great: layer, layer, layer, but layer complimentary kick drums (don't layer three low end kicks; try one that's low, another that's mid and punchy, and maybe a thin one on top, maybe even a thick snare with a low-pass filter to give it some crunch)
And yea, the kick should sit right "on top" of the bass line, so to speak.
Everything should have its own rock to sit on.
-Hypno