The consenus seems to be in this and the downhill put thread is to keep the disc parallel to the ground. The natural tendency is to use a putting style for flat putts, but this tends to orient the disc too high or too low on downhill or uphill putts, respectively.
Also, use a straddle style, as Ken Climo does on these, I guess because getting one foot ahead of the other places you in an awhward position--unless you are using a Ron Russel throwing position, as Blake suggests.
Too practice, find a tree on a hill in your local park (doesn't have to be a disc golf park), and start throwing all around it with different styles.
Nevertheless, I have problems, too. For some reason, one basket is buried in the ground about 5 inches shorter than it should be, so it becomes like a downhill putt. It drives me insane, I can't hit it at 15 feet, and sometimes when I do, perfect putts bounce off the chains and drop out. Putts hit the chain in the center with a strong sound, no glancing, and then I stare at it on the ground. Arrrrrrrghhhh.