Well, it's actually kinda hard to explain in words - you need to hear it. In fact, the instrumentals of the A Tribe Called Quest album The Low End Theory and most of DJ Hi Tek's work are great examples.
The low end theory is basically filtering out the high-to-midrange frequencies [basically isolating the low end] of a bass sample [or any sound that could potentially be a bassline] until it produces a warm "bounce" that doesn't sound muddy. DON'T boost the low frequecies, just cut out the others, boosting ends up with unnecessary gain in the audio.
The turntable cartridge utilized by most hip hop producers, the Shure M-447, tends to be low end heavy in its initial sound, so that also plays a role. I personally prefer a more balanced sounding cartridge like the Grado Prestige series, so the filtering is more crucial in my position.
Just experiment; if you mess up, you might come up with something totally original.