Some of the poker books
have indicated that most people that try to cheat do worse than decent poker players.
Collusion is the most common form of cheating online, and none of us would know that 2 of their opponents happen to know each other and are chatting on something like MSN. However, certain plays can then become suspicous, and it's the reason why there is software that will flag your account if you are regularly at the same table as another player, so that they can investigate whether there are any signs of collusion (e.g. folding when the colluding 'opponent' had a monster, or helping cap the betting with a weak hand). A less easy tactic to stamp down on is soft-playing - simply not crippling a friend who you're playing against, so that they have a better chance of doing well in the tournament.
In reality, while the folding to a better opponents hand saves a bet, it's a bet that would go to your friend, and it reduces pot odds
and may encourage others to fold as well, costing your friend more than it saves. Helping cap the betting with a weak hand is I suspect am much less common approach, but that is probably the more profitable for the both of them put together. If the cheaters do not have a very solid understanding of poker in both pot odds
and psychology, then there is a risk that their efforts to cheat will actually cost them money even if they do avoid being detected, so against the most likely forms of cheating online I don't worry too much, because those trying to cheat in general probably don't have the necessary poker experience to ensure it truly benefits them.
As for cheating myself, no - I've learned how to do false shuffles and the like in home game, and considered what strategies should genuinely give an advantage online, but despite that I haven't cheated, and don't plan to start - it's more been to reassure myself about chance of spotting cheating, knowing what to look out for. Since I'm making a profit anyway, I don't see the need to do things dishonestly.