What seems to be really crucial to me is feedback paths.
What? Let me explain:
(I'll use the analog mixer as an area for exploration, but you can accomplish the same thing (almost - but that's another story) with a digital/software mixer.)
For example, you set up a delay on one channel. You set up an Aux Send that sends to this delay channel. Now, on the delay channel, you insert a bit of hi-frequency filtering and maybe a touch of overdrive (mda combo[/b[] is nice for this, as a free VST plugin). Rather than turning up the feedback control on the delay, turn up the Aux Send on the delay's return channel. This way, the delay passes through the filter/overdrive on each repeat, so gets duller and more distorted with each repeat. Sounds much more organic than a straight delay (which sounds pretty sterile in comparison).
Use other stuff on the delay channel - phaser is nice, or a flanger without much feedback. The idea is to change the sound with every repeat.
For more complexity, set up a spring reverb on one channel and a delay on another. Set up Aux Sends going to each. Send the output of the delay into the reverb, and the output pof the reverb into the delay. Use a mono delay and a mono reverb, and pan them differently. Nice evolution and organic goodness.
Automate your send levels or (better yet) play them by hand while recording. If you want, you can cut 'n paste the best pieces from this experimentation, and use 'em again within your tune.
If you're using a digital mixer, make sure your mix controls are at 100% wet. (No time to go into this now, but suffice to say that it'll keep you free of comb-filtering thinness.)
Just a few ideas I've found useful.
-Hoax