One thing that helps for me is that if I win the first set, I start the second telling myself that I want to win the second set by a better score than the first.
I think that this is a bit easier for the mind to accept than the standard advice to "pretend you lost the first set." I know I didn't lose the first set, so my mind isn't buying it.
Ah, but if I win the first set 6-4 and set as a goal not letting the opponent get more than three games . . . for some reason that seems to help me continue to attack rather than sit on my (temporary) lead.
-- Random Error Generator, Version 4.0
-- Master Moonballer