Changing Up Your Heads Up Game
If you're a heads up poker player, one very important skill is the ability to switch gears during a heads up poker match. One of the easiest ways to beat a player heads up is to take it slow and pick up their tendencies, and then ramp things up and take advantage of those same tendencies. However, to defend against this you should also make constant changes to your strategy. By switching your plays during the match you'll keep your opponent off balance, and ensure that he/she don't have any reads on your game.
Preflop Play
The first situation that requires varying your plays is preflop play. When you're the dealer, you're likely to raise more often, and when you're the big blind, you're likely to check/call more often. Also, when you have a premium hand, you might slow play it the first time or two just to be deceptive. However, over the course of the match, you can expect to have a couple big hands, and you should play them differently just to throw your opponent a curve ball.
For example, if you have AA early in the match, limp in to feign weakness, and end up winning a decent pot where you show the hand down, you need to remember this and adjust accordingly next time around. If you get another big hand, I'd recommend raising it preflop so your opponent will think you have a decent hand, but not a monster like last time. Then, the next time you have a big hand, no matter what you do your opponent can't peg you on a hand because you've already shown that you'll play it in different ways.
Flop Play
Besides just switching up your preflop play, you should also adjust accordingly on the flop. One common mistake that players make is to always check the flop when they hit it, and bet the flop when they either miss or get a very small piece. To counter this you should always alternate between betting and checking your big flops, and betting and checking your weaker flops.
Also, when your opponent fires into you, you should consider how you played before. If you always just call a bet with top pair, consider raising once in a while to throw him/her off. Another situation you should vary your play in is when you're bluffing or semi bluffing. Don't fire out into every single flop when you miss, because it becomes very predictable and will give your hand away.
Value Bets vs. Over Bets
Another spot where you can really switch things up is when you have a solid hand on the river. Some players will always value bet here, and make a bet of roughly 1/4 to 1/3 of the pot. This becomes very predictable, and your value bets will soon turn into value-less bets. Because of this you should throw in an occasional over bet that is pot sized, or even check once in a while in hopes of inducing a bet by your opponent, which you can then raise. For more information on this topic, watch this SNG poker video which describes changing gears while heads up.