i do service line groundies first, occasionally have done easy volleys at net, but mainly the groundies. I prefer to spend a while at the service line till i am hitting well and with good control and top spin and using good footwork and a good follow through. At that stage i am pretty much warmed up, if i dont get to do that, then i am not at 100% rallying from base line, not straight away that is. I find partners get bored quickly or just dont have the control to maintain this kind of warm up. The only guy i found who did maintain it and showed good form went on to beat me, none of my other partners have. Maybe a message in that somewhere.