Dirty tricks I pull against the CPU:
Hard tackling and always closing down against players with low bravery, natural fitness or high aggression to scare them out of 50/50's, injure them or provoke them.
In the Italian league, sell 50% of a player when you're with a wee team in Serie B to a big team, but retain the rights so he stays with you. Don't play him for a bit and his value drops. Buy the 50% you sold to the big team for a low LOW price. Might even be able to sell him on again if he's young enough (and repeat!).
When you've sold a player with a next transfer fee percentage clause, put him on your shortlist to see when another club is bidding to buy him. Stick a bid in thats higher than what they offered, and wait for them to re-bid. A good way of inflating the price!
Any others?
Or am I just a really sneaky bastard?