In heads-up no-limit hold'em, aggression and position are Important and you should be raising from the button with a very wide range of hands. Your aim is not just to put maximum pressure on your opponent and build pots, it's also to develop the opportunity to put him to tough decisions later in the hand.
Out of position, you will need to play much more , defending only with hands that you can play with confidence, like pairs, suited connectors, Broadway cards, and high or suited aces.
You also need to balance your ranges and attempt to seize back the momentum in a hand by re-raising frequently against a player opening a lot of hands from the button.
Against aggressive players who re-raise frequently, you will need to combat this through a mixture of four-betting with marginal hands, calling with big hands to trap and calling with more speculative hands. You should also be prepared to go broke with a decent top-pair type hand, a big draw or even just a big bluff