Respect your opponent's ability
Understand that your opponent probably had a reason for playing where he did, and don't blindly ignore moves that appear stupid at first. Additionally don't disrespect your opponent's ability by playing moves that rely on them severely screwing up to be effective, such moves are insulting and highly irritating.
Finally, always play honest moves against a weaker opponent, avoid trick plays. It is your duty to use your strength to show them the correct way of playing.
Yesterday I played a nine-stone teaching game against a beginner who won't remain at beginner level much longer. At a certain stage I said: "Look, I'm going to invade here (sansan), although it is a move that should not work. But if I continue to play properly, I lose this game (I was 30 points behind). Maybe my invasion will succeed, but it will be a nice exercise on how to punish an overplay."
And so we went on. With some hints, the whole corner developed into a ko, and with the compensation of the ko threat, and the yose, I closed the game down to a two-point loss. He was very pleased.
So, trick plays are very instructive teaching tools. If however, you use them only to crush weaker players, you're doing damage to your own game and to the game of your opponent.
I don't 100% agree. I find many beginners are way too respectful of the stronger player's moves. I think you need to be able to punish overplays if you want to get stronger, so I regularly play moves I know are bad when I play beginners.
The point is to set a problem for them and see if they can find the correct move. Of course there wouldn't be any point in this if you didn't analyze the game afterwards and explain the situation...
Coyotebd? I'm a good teacher then, as I often make horrible, terrible mistakes in handicap games. I then make noises and explain how I made such a terrible mistake. It gives the person I am playing against a chance to spot the mistake.
If they don't get it I point it out to them once they've played. Usually if I make the same mistake later they catch me before I've even seen it myself.
SnotNose: I typically avoid trick plays and, more generally, most plays I can see are bad (with correct play by opponent). That is, I try to win handicap games (as White) based almost exclusively on mistakes by my opponent that I didn't induce. My reason for playing this way is that I'm thinking long term. I don't want to practice bad moves (practicing bad moves is worse than making them accidentally). Nevertheless, I lose a lot of handicap games as White. I don't mind, except during tournaments, at which times I wonder if I should practice a few trick plays just for tournaments...probably not. That would be inconsistent with my philosophy and for only vain reasons.
kvas: I have a friend (2d) who is playing the sort of Go that i would call hooligan style. When I play him (i'm 10k) he gives me 8 or 9 stones handicap and he can't resist the temptation for some overplay (well, he even uses it in a regular game). I find games against him rewarding, as I can develop my fighting skills very well, and if I manage to punish his overplay, it raises my confidence too. Wouldn't say that I like his style very much but for for developing ones tactical skills he's one of the best opponents.
Jade: As an addendum to this, do not assume your opponent will make the same mistake twice. It is very tempting, when an opponent ignores a good move, to play under the assumption that they will continue to miss it (and thus to ignore it in favor of smaller moves).
Alex: This is probably my biggest weakness. No matter how hard I try, I can't seem to break my own bad habit of losing respect for an opponent who plays ridiculous moves. It's particularly a problem when playing online, because of the crude "Internet style" found on most servers. My opponent will play abysmally during the fuseki and early middle game, giving me a large lead, at which point I start to think "man, this guy is weak" and stop paying attention. But of course, these players make up for weak strategy with stubbornness and strong tactics. If I'm paying attention, I can usually thwart their rabid late-middlegame attacks, but quite often I slip up because I wasn't giving him/her enough respect.
ilan: I have been thinking about this phenomenon recently. I have long thought that every game (go, chess, pool) has its particular rythm that one must come to terms with and that the above phenomenon is particular to "go rythm". In particular, in a typical game of Go, there can often come a very rapid transition in which strategical and intuitive decisions give way to purely tactical considerations. That is, you often see that groups which were connected or alive suddenly fighting for connectedness or life. From observation, this happens at every level, since I have seen a number of 7d games in which groups suddenly get disconnected or die suddenly near the end. Therefore, one must be prepared to quickly change hats from master strategist to master tactician. In some sense, this is unavoidable, because one can argue that the purpose of superior strategy is to give you a better chance of winning the tactics.