I'm probably breaking with tradition here but I've never been a great fan of the riffle force. I find that it needs impeccable timing and skills to look perfectly convincing. I have gotten away with it but I've never liked it much. Why use an alright force when you can use a much better one?
My current favourite is the spread cull force which looks similar to the Classic force except that any card can be touched and yet a card can be forced. Very good force.
Cross cut is a very good one to use when you want the illusion of non-interference on the part of the magician. There are a couple of subtleties for it, the best one (can't remember where I got it) being to place something on top of the deck once it has been cut. It makes it look very very convincing. Note: don't be worried about using the cross-cut force. People really are that, er, thick?
Another favourite of mine is the Bottom Bluff force from Smoke and Mirrors. It's a force from the same genre as the cross-cut (i.e. obvious but fools
everybody) and is just so fun to do. It even works on magicians who haven't read Smoke and Mirrors!
Those two (three) are probably the only forces I use. They work quite well for anything I need to do
Kati