If the blinds are high, you have a potention raising hand if you have pocket 3s or higher pairs, suited king 4 - 2, suited queen 7- 5, suited jack 8, suited jack nine, unsuited ace 2, unsuited king 6- 4, unsuited queen 9, unsuited jack ten, or unsuited jack nine.
Remember, in heads up, any pair is a big hand and u should push strongly. Also, any ace is a big hand. If you get dealt KK, AA, or QQ and youre opponent is aggressive, just limp in when youre in the small blind, and check if youre in the big blind (and hope the flop hits your opponent but gives him the 2nd best hand).
Also, think of flopping middle pair in heads up as the equivalent of flopping a set at a full table.