My advice on when to fire second barrell...

First, When NOT to fire:
-- When you are OOP, and have little chance to improve your hand on the river.
-- When villian doesnt fold when he should.
-- If you have a bad image (loose)
-- if there is more than one opponent

There are 2 main things i consider:

#1 ~ the quick call
--If the villian calls your flop Continuation Bet very quickly, this usually means that villian has a good but not great hand (or a draw). It is often second/third pair or an underpair. A gutshot. Top pair is also possible.
-- I fire again about 50% of the time when this happens depending on the turn card, who the villian is and what cards I have.

#2 ~ the scary turn
--Overcards. The best ones are an A or K.
--Flush or straight cards. They can work but villain could have called the flop with a draw. However if villian has a 1 pair hand firing when a draw hits usually scoops the pot.


**I fire almost 100% when both #1 and #2 occur**

Also, i rarely fire when out of position. Unless I have outs and #1 and #2.