First things first you select either the first beat or the first two beats of the section you want to build up into and bounce down a copy of this (you just need one or two beats - the picture below I just went for one).
Ok, import the bit of audio into a blank audio track and add a nice long reverb (try starting with 100% wet, sometimes this trick works better with a little bit of dry tucked in there are well), now bounce a copy of this snippet of the drop with the long reverb out. In this picture I used a nice long reverb from Logic's Space Designer, but any nice nice sounding reverb will do the trick.
Now open up this new copy with reverb up in your sample editor of choice and reverse it (I'm using the one built into Logic), so it looks something like this.
Right now we're ready to position it so that it builds up into the drop (I had to chop a little bit off the front of my reverse reverb build up because it tailed off a bit) leaving you with something like this
I like this technique because it doesn't sound quite as rinsed as the band passed white noise sweep. You can always experiment with further processing the build up (maybe a bit of subtle flanging or some other stereo modulation effect).
Edit: Here is an Audio Example of the reverse reverb build up