http://smashboards.com/showthread.php?t=194351
Taken from that thread:
Fast-fall it whenever possible, and do it MUCH earlier than you would expect. It is in itself a laggy move, and furthermore, a meet-me-half-way move. Begin pretty much as soon as you start fast-falling, and it should work better.
Also keep in mind you CAN combo them.
Sour > Sour > Sweet works well. I have a vid of that somewhere.
Or sour > sour > footstool.
Please, please search.
EDIT:
Oh, I forgot. Check out this guide.
http://smashboards.com/showthread.php?t=179565
There are a few combos and tricks that help you land spikes. One of them is even a 0%-to-Death combo.