Good health is key. You want your females big... NOT fat. Fat females have a tendency to slug out.
One thing I've done that has had 100% success rate for me so far is I do not palpate my females. I just don't feel like I'm very good at it, and I fear that I might do more damage than good trying to figure it out. Lots of people palpate and have success, I don't and have had tons of success... so to each his own.
When it comes time for all the pairing and whatnot I don't touch my females unless I'm cleaning their tubs. I think removing as much stress as possible helps a bunch too.