Start overbetting the pot?

[triplc]

7-handed 10+1 SnG on Stars...I've got T1430 and the blinds are 15/30. I get 99 in the small blind.

UTG limps, all fold to Button who raises to 90. I call UTG calls. Three to the flop. 300 in the pot.

Flop comes 854 two spades. I bet 300, UTG calls, button folds. Turn is a 7. Pot is 900, UTG has about 800 left in his stack, so I decided (wrongly...I admit) to underbet the pot (300) and let him react to a bet of 40% of his stack. I'm thinking he might have TT, JJ or trips, but want to take a stab anyhow. He calls. Turn comes 9 of spades. Hit my trips, but a flush and (still) straight draw on the board. I check, he pushes in. I call (mistake #2...or maybe mistake #3, #4, #5 in this hand...someone please tell me), and he has J4s for the flush. I'm crippled and go down shortly thereafter in 6th place.

Analysis of the individual hand is appreciated, but I'd also like comments about overbetting the pot on the flop. I rarely (maybe never) do it unless I'm pushing in and usually make a pot sized bet when I'm protecting my hand.

Thanks in advance.

CCC