I'm going to talk about 2nd barreling. For me, 2 barrels are a a essential part of my game because i c-bet so much. I c-bet like 90% of the time in a HU pot, so when people realize that they start calling my flop bets light. This means i have to throw in some two barrels.

The most ideal spot to two barrel is on a low, dryish flop, when the turn card comes a high card, like this.

POKERSTARS GAME #9667114224: HOLD'EM NO LIMIT ($1/$2) - 2007/04/29 - 14:26:03 (ET)
Table 'Theobalda' 6-max Seat #5 is the button
Seat 1: SmutsHarry ($350.85 in chips)
Seat 2: BowWowRomeo ($136.70 in chips)
Seat 3: kawelljd ($201.50 in chips)
Seat 4: IMSUPERCREAL ($203.10 in chips)
Seat 5: iamtufts ($236 in chips)
SmutsHarry: posts small blind $1
BowWowRomeo: posts big blind $2
*** HOLE CARDS ***
Dealt to IMSUPERCREAL [Ac Th]
kawelljd: folds
IMSUPERCREAL: raises $6 to $8
iamtufts: folds
SmutsHarry: folds
BowWowRomeo: calls $6
*** FLOP *** [5h 8h 3d]
BowWowRomeo: checks
IMSUPERCREAL: bets $10
BowWowRomeo: calls $10
*** TURN *** [5h 8h 3d] [Js]
BowWowRomeo: checks
IMSUPERCREAL: bets $25
BowWowRomeo: folds
IMSUPERCREAL collected $35.20 from pot
*** SUMMARY ***
Total pot $37 | Rake $1.80
Board [5h 8h 3d Js]
Seat 1: SmutsHarry (small blind) folded before Flop
Seat 2: BowWowRomeo (big blind) folded on the Turn
Seat 3: kawelljd folded before Flop (didn't bet)
Seat 4: IMSUPERCREAL collected ($35.20)
Seat 5: iamtufts (button) folded before Flop (didn't bet)

POKERSTARS GAME #9420277816: HOLD'EM NO LIMIT ($1/$2) - 2007/04/14 - 16:00:58 (ET)
Table 'Ankaa V' 6-max Seat #4 is the button
Seat 1: Shanyu ($366.60 in chips)
Seat 2: ROCKTITE ($88.30 in chips)
Seat 3: umpa21 ($202.35 in chips)
Seat 4: IMSUPERCREAL ($174.65 in chips)
Seat 5: millsy101 ($226.95 in chips)
millsy101: posts small blind $1
Shanyu: posts big blind $2
*** HOLE CARDS ***
Dealt to IMSUPERCREAL [Js 7s]
ROCKTITE: calls $2
umpa21: folds
IMSUPERCREAL: raises $8 to $10
millsy101: folds
Shanyu: folds
ROCKTITE: calls $8
*** FLOP *** [5s 3c 5d]
ROCKTITE: checks
IMSUPERCREAL: bets $14
ROCKTITE: calls $14
*** TURN *** [5s 3c 5d] [Kh]
ROCKTITE: checks
IMSUPERCREAL: bets $32
ROCKTITE: folds

Other times to do it are on J, Q or K high dry boards:

POKERSTARS GAME #9182606636: HOLD'EM NO LIMIT ($1/$2) - 2007/03/31 - 14:49:58 (ET)
Table 'Danjon II' 6-max Seat #4 is the button
Seat 1: adp0211 ($41 in chips)
Seat 2: i reraiz ($198 in chips)
Seat 3: IMSUPERCREAL ($244.90 in chips)
Seat 4: DangleSauce ($203.65 in chips)
Seat 5: Sir Joey J. ($71 in chips)
Seat 6: Bennard ($200 in chips)
Sir Joey J.: posts small blind $1
Bennard: posts big blind $2
*** HOLE CARDS ***
Dealt to IMSUPERCREAL [8h As]
adp0211: folds
i reraiz: folds
IMSUPERCREAL: raises $6 to $8
DangleSauce: folds
Sir Joey J.: folds
Bennard: calls $6
*** FLOP *** [6d Jh 3s]
Bennard: checks
IMSUPERCREAL: bets $10
Bennard: calls $10
*** TURN *** [6d Jh 3s] [4s]
Bennard: checks
IMSUPERCREAL: bets $28
Bennard: folds
IMSUPERCREAL collected $35.20 from pot
*** SUMMARY ***
Total pot $37 | Rake $1.80
Board [6d Jh 3s 4s]
Seat 1: adp0211 folded before Flop (didn't bet)
Seat 2: i reraiz folded before Flop (didn't bet)
Seat 3: IMSUPERCREAL collected ($35.20)
Seat 4: DangleSauce (button) folded before Flop (didn't bet)
Seat 5: Sir Joey J. (small blind) folded before Flop
Seat 6: Bennard (big blind) folded on the Turn

on K, Q, or J high boards, it's most likely someone is calling you with a mediocre hand that won't call you on the turn. Another good time is when you pick up a draw on the turn in a decent fold equity situation. However, i have to say that i don't two barrel that often, in fact, it took me a few minutes to even find an HH of me two barreling in my database. Don't make to big of a habit out of it.

For fun, i'll also talk about three-barreling. This is definitely something that should be used sparingly! The hand below is a great three barreling situation:

POKERSTARS GAME #9472245323: HOLD'EM NO LIMIT ($1/$2) - 2007/04/17 - 16:45:02 (ET)
Table 'Tokio' 6-max Seat #4 is the button
Seat 1: IMSUPERCREAL ($290.85 in chips)
Seat 2: schickli ($197 in chips)
Seat 3: Stimpy444 ($194.35 in chips)
Seat 4: dNoUnTkSe ($213.30 in chips)
Seat 5: ELROYCASH ($77.75 in chips)
Seat 6: Apothecary04 ($160.55 in chips)
ELROYCASH: posts small blind $1
Apothecary04: posts big blind $2
*** HOLE CARDS ***
Dealt to IMSUPERCREAL [8s As]
IMSUPERCREAL: raises $6 to $8
schickli: calls $8
Stimpy444: folds
dNoUnTkSe: folds
ELROYCASH: folds
Apothecary04: folds
*** FLOP *** [5h Qc 9h]
IMSUPERCREAL: bets $14
schickli: calls $14
*** TURN *** [5h Qc 9h] [Jd]
IMSUPERCREAL: bets $34
schickli: calls $34
*** RIVER *** [5h Qc 9h Jd] [5d]
IMSUPERCREAL: bets $234.85 and is all-in
schickli: folds

The opponent here is a standard TAGG who's a little more on the tight side, he is a reg. First off don't try and threebarrel bluff a non-reg, it won't work enough of the time. The reason that this is a good three barreling situation is because the board is very draw heavy on the turn. This guy gives away his hand by not raising the turn, because with a strong hand it's highly unlikely he wouldn't raise. At this point, he probably has some kind of draw or at best, TPTK. By overbetting, i make a bet where he can't call with a hand like that! The turn bet is questionable but ok.

Obviously, if this guy ever catches me doing this, i'll have to adjust. But as long as i don't showdown i can do this every once in a while (and by that i mean relatively rarely).