I've read of some folks actually dumping media into a tub of water to soak - not sure if it actually improves anything. But what DOES definitely improve things, is dryer sheets. Tear the dryer sheets into approximately 1" squares, and put them into the tumbler along with the media and brass.
Tumbling Compatibility List
Ideally, your entire batch of brass would be of the same caliber. This is mainly so that you don't have to sort it afterwards. However, there isn't any harm in tumbling multiple calibers of brass together, as long as they all have the same neck diameter. This is important because if a piece of brass manages to lodge itself within another piece, neither piece will get thoroughly cleaned. Below are some calibers you can safely tumble together:
There are also plenty of other calibers which aren't as compatible with each other. 40S&W is notorious for causing trouble - it will readily slide into 45ACP, and 9mm will slide into the 40S&W. You really don't want to play with the brass turducken which results from this combination.
-- SeanNewton - 09 Mar 2008