Poker is unique in many ways. Unlike reporting taxes to the IRS or anything having to do with the opposite sex, lying is actually encouraged in poker. They use the cute little term "bluffing" but it is the same thing. Bluffing against a field of players is one thing, but it is completely different when you are in a heads up game and you're the sole focus of your opponent.
Most people who are good at heads-up poker bluff infrequently. The semi-bluff is a weapon in their arsenal that they are more likely to brandish. The reason for the infrequent use of the bluff is that your opponent is going to figure it out and lower their threshold for what they consider a playable hand. In essence, they will start playing lower quality hands in order to stop your bluffing.
If your heads-up game is a marathon rather then something that is only for a relatively short amount of time, you may want to consider bucking conventional wisdom and using the bluff on a frequent basis. The reason for this is you can use the bluff to force your opponent to lower the quality of hands they are willing to play. Once they start doing this, you stop bluffing and take advantage of their low strength cards they are playing the hand with.
In the short term a bluff can be used to win the occasional pot, but if you take a long view of bluffing as a tool of manipulation, you can control your opponent's behavior.


