33 flops bottom set on coordinated board

Your just lucky the CO doesn't like money or this hand would have cost you a couple more BB. I think the general theory is that until someone makes you stop being aggressive (by c/r or re-raising you) bet your strong hands.

The best way to protect your hand: don't cap the flop, check-raise the turn (if the turn wasn't a spade you could bet out for value), if you get re-raised you know you're up against a flush and can fold if you don't fill up on the river, and if you do fill up on the river, you should be able to extract extra bets. The way that you played it, the only information that you have suggests your hand is good, if a player is timid with a small flush he will call you down though. I stopped playing 33 UTG because of hands like this.