From what I have seen in articles and forums, there is an over-emphasis on blind defense in no-limit play. This is somewhat understandable. Defending your blinds in limit hold'em can be a very crucial element of overall success. Your blind is worth half a big bet. If you are grinding out a 1BB/100 win rate, half a big bet is pretty significant. Tournaments are also a game in which blinds eventually become a crucial part of the game. Most recognize that stealing blinds early in a tournament is not worth the risk, but in mid to late play, stealing blinds can be very lucrative. Hence, tournament advice stresses the importance of protecting your blinds.
Now in no-limit hold'em these considerations do not apply. Half a big bet is not a large part of the typical no-limit win rate. Blinds are not increasing regularly putting pressure on your stack size. They remain constant and though not quite as small in proportion to your stack as initial blinds in a tournament, they are still relatively small. Losing your blinds won't affect your ability to finish in the money, since there is no money aside from what you win or lose in each individual hand.
Defending your blind against a steal is difficult in large part because you will have the worst position for the remainder of the hand. Many people defend their blinds with marginal hands, like an offsuit K-10. There is some logic here. You might expect the stealer to hold a lesser hand, and so you call with a lesser one yourself. Remember, however, that you are supposed to play tighter in early position than in late position. Why is that? Because you will have the worst position for the remainder of the hand. Typical playing hand requirements in a full ring game drastically limit the hands that you should play in early position. This is done to protect you against bad position. A hand as good as A-A can make up for bad position. Now if you were to follow that logic, what conclusion would you come to about defending your blind with K-10? You should adjust against the raising range of your opponent, but don't forget about your poor position. My advice here is centered on the poor position you will be getting, and this leads me to recommend a very conservative blind defense strategy.
Let's begin by considering the type of player you are up against. If the raiser is a known blind-stealer (i.e., you've seen him/her raise in stealing position too frequently to have a real hand every time), you have two defensive plays. One is to hammer back and the other is to slow play. The hammer is the solid play. One tendency among blind stealers is to call the reraise. This is done to save face ("Hey, I've got a real hand here, I'm not stealing, honest!") and to try and bust you by hitting two pair or better. If the stealer misses the flop, however, he/she will be mucking. The slow play defense is generally only done with a big pair A-A down to Q-Q. Here you plan to bleed the stealer by letting him/her continue the steal attempt. It is particularly effective against a stealer who likes to bet the pot on the flop and 3/4-full pot on the turn.
The hands I recommend considering a defense with are rather limited - any pair better than 8-8 along with A-Q and A-K. Early on I recommend slow-playing only with A-A and K-K. If you have been at the table for several orbits and the stealer has stolen your blinds several times, then you can consider Q-Q for a slow-play, because all your folding has induced the stealer to steal with even more hands, perhaps any two. An A or K will flop on you one-third of the time, but if you are up against a loose stealer, then when this happens you can still play confidently. If a dangerous flop hits, say one with two spades or two consecutive cards, then a check-raise on the flop is probably your best play. If a scare card hits the turn, you will be in a tough spot both cards-wise and position-wise. Consider a brief example. The stealer raises 3 big blinds (bb) and you call with A-A. The flop is a drawing flop and you check over to the stealer who bets the pot (6.5bb). You now raise and the stealer mucks. You've made 9bbs (minus .5bb for rake). That one play makes up for nine folds.
I recommend a hammer approach for all the other hands I listed in the last paragraph. These are all vulnerable hands that you do not want to play out of position. 8-8 is a marginal play, but if you have been at the table for a while, and the stealer is loosening up on the steals, then you still stand a good chance of having the best hand after the flop. 9-9 is similar early but gets better later. Now as I said above, many stealers will go ahead and call and then fold without hitting the flop hard. In general then, you will tend to make a fair bit from the hammer play.
I recommend folding all lower pairs and worse aces. Small pairs need to flop sets, and as I will show, that doesn't happen enough to make defending with a call a good play. Weaker aces as always are trouble hands unless you flop big. There's no rule against the stealer actually having A-Q. You will be paying dearly with you're A-J in those cases. Moreover, you will only be hitting the flop one-third of the time and check-folding to the stealer's continuation bet the rest of the time. (You could check-raise the flop, but that play is more advanced and you should hold off on that one until you feel comfortable with a basic defense strategy).
You should not call with small pairs hoping to make a set against a stealer because the implied odds are not good enough. (A loose-aggressive player is an exception here, since they may fire three barrels). You will miss your set 7.5 times for every one time you hit. If the stealer is raising 3bbs, you will lose 3bb 7.5 times for a total of 22.5bb. You need implied odds of 22.5bb to make up for that loss. Assume you hit on a safe board and the stealer bets full pot. You call and the stealer fires a second barrel of 14bbs. You now raise and the stealer mucks. You make 3bb+6.5bb+14bb for a total of 25.5bb. This is 3bb more than your previous losses, but the rake will likely be one or two big blinds, so the win is only just enough to cover the losses. Now for more bad news: you won't always have that safe flop. Sometimes you will need to check-raise the flop, winning only 9bb. When this happens you dig yourself less than halfway out of your hole. The problem with a calling defense with small pairs is that you have very small implied odds against the stealer. The stealer doesn't have a big hand. He/she is just going to fold when you bet or raise.
Against non-stealers you should simply play your normal game. Call with the hands that you would call with out of position. Small pairs do have good implied odds against these opponents. Similarly, loose aggressive opponents bring good implied odds as well. Defend your blinds against these opponents just as you would outside the blinds in bad position. I have been assuming that you are defending your blind heads-up, but if a player between you and the stealer calls, then you should pretty much eliminate the slow-play. Take your big pair up against one opponent if possible. I still think small pairs are negative EV here too and would muck them, but would hammer with the other hands.
One final note: This is an exploitable strategy, but most strategies are. You can make it more difficult to exploit by throwing in some wrinkles, such as occasionally hammering with air or continuing to slow play on dangerous boards. If you are playing at low stakes, however, such as 25NL, the upside of tweaking the strategy is pretty low. The number of players who are watching your play with the kind of attention necessary to determining what your strategy is and how to exploit is a micro percentage of the total players. If you think a player has your number, don't play with that player. Move to a different table. As you move up in limits you will eventually need to modify this basic approach, but as a starting point, it should be fine.
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