For whatever reason I am typically the more vocal partner and I always go the supportive route.
"Alright lets get this next point"
always complimenting their shots
even on a miss "good effort"
If they are serving bad I say "Relax just get us in play and we will take care of it in the rally"
and I often point out the other teams weakness to my partners and comment how they can't handle the pressure from some of my team mates shots... and anything else to help build confidence thru the match.