Opponent could have a flush...?

Hi,
I seem to often get caught when my opponent has hit his flush... let me give you an example:

I am big blind, I have a very average hand but it is folded to me excepted for one limper... I raise with say Q9o...
I hit my 9, top pair, on the flop and there are 2 diamonds on the board, I am 1st to act, so I bet the pot... opponent calls, turn comes J, no diamond... I bet 3/4 pot on the turn... unlikely that my opponent called with a J, river comes low diamond... im to act 1st, should I check? which could let my opponent bluff that he has the flush? what i did was bet about 1/4 pot... my opponent min raised me... I had to call for value...
Should i have folded? what are strategies that one could use when not sure if opponent has a flush or not...

I would greatly appreciate help with this...
I know i played this hand badly... is it just a disipline thing, or have i done something else seriously wrong?

Thanks

Sounds like you played it well enough, but the other guy was willing to chase and win or lose quite a few chips if he didn't hit his flush. It's so hard to judge in this situation. What would you have done if you were in HIS shoes? I would have probably chased as well, unless you had raised a whole lot more. Tough one!

I probably woulda chased too, he did have the K-high flush in the end... it is tricky, cos I don't think he woulda folded... but maybe I should have just checked the river... and then folded, but, i find it so hard to fold in that situation. don't want to be bluffed every time there is a possible flush on the board, I guess, I shoulda just taken the calls as him chasing and check folded on the river.

I agree with the rest that called this a tough one. I am no pro by any means....a work in progress at best but I probobly would have done the same. This kinda thing has a lot to take into account. Position, Chip stack, the agressiveness of oponnant, what level the tourney is at etc. When I get the feeling I might be beat {or about to be} I try to remember:
IF THERE IS A HAND THAT CAN BEAT ME SOMEONE PROBABLY HAS IT

oh yeah when possible ALWAYS use the "note on player" option to track known chasers loose/tight players etc.. It might help the next time you play him/her. Good luck.

Thanks prankish imp, just lettin u know... this was not a tourney... was full ring cash game...