During the early part of a heads-up Sit & Go’s you need to think a lot about position and play many hands, with the effective stacks getting shorter due to the blinds going up, you’ll need to revise your game significantly.
With 20-30 big blinds your strategy shouldn’t change that much from before, although you might now want to lower your standard raise size to 2.5 big blinds, especially against an aggressive opponent who likes to re-raise all-in a lot. Re-raising all-in yourself is also a good strategy against a player who raises the button too much, as they will have to fold the vast majority of the time and it’s a great way to negate their positional advantage. You may also want to consider limping on the button occasionally at this stage to slow the game down and avoid getting in too many big pots.
Getting short-stacked
When the stacks fall into the 15-20 big blind range the above guidelines still apply, but even more so as re-raising all-in becomes a less expensive option. Therefore you need to adjust your ranges for raising, folding and limping on the button even more significantly, depending on how your opponent plays, and beware of losing even a couple of medium-sized pots as this will put your chances of winning in peril. At this point calling raises and check-raising all-in on the flop out of position becomes an excellent idea if you hit a decent hand, as the stack sizes are perfect for doing so. This is especially true if your opponent’s range for raising and continuation betting is still quite wide.
With a short stack of below 15 big blinds, and especially as you fall below ten big blinds, aggression is the key and your best option is usually to just move all-in or fold. Most players tend to play too tight at this stage, so remember that whenever your opponent folds you are winning 1.5 big blinds, and even when called you usually have at least 25% equity in the hand. For this reason shoving hands that don’t look that great like J-3 suited or 9-7 offsuit is fine with a ten big blind stack.
If you want to find big blind values for all hands then the The Mathematics of Poker written by Bill Chen and Jerrod Ankenman is a perfect place to start.