Sorry for the sidetrack, but when I read "even if you know it will bounce" from Draugs reply, it reminds me of my FAV thing to do in a game to really get the goat of an opponent: Supporting one half of an opponents self bounce.
IE: During the fall phase Mars is empty, but my opponent has an army in spa and pie (and a build coming next turn and he wants to build in mars)... and I have a fleet in lyon and an army in venice. What would my opponent like to do? Bounce spa and pie in mars (spa-mar, pie-mar) so that it is empty for him to build, but he still has his advantage in pie. So what do I do?
Lyon S pie-mar, ven-pie.
Now pie has 2 on mars, spa has 1, so pie actually succeeds in moving to mars (even if my opponent didn't want that), and I gain pie. Now he can't build in mars and I took his advantageous position from him.
Note that this only works on two units bouncing in an empty province. You can't support an opponent's unit into one of his other units and succeed (if, say, mars had a unit in it already, I can't force him to displace a unit with his own attacking unit), but it is a nice trick that a lot of players don't consider it when making moves.