I tell them either - good luck or you can win this game.
If it's a game I couldn't care less about I just say have fun. Or if it's second half of some final. Fun is always important for finals.
if I play against a rival I tell them to do it for the fans!
If I want a trophy really really badly, and most of them have me as favorite I tell them that I expect them to win that trophy. Doesn't work if they don't like you though.
Other than that, halftime talks are mostly different for different players. If my star player is playing a 7 and the rest are in the 8. I tell him to shape up or get out (usually get's him mad, but helps

)
Other than that, I randomly click.