I use an orange plastic cone. I put it about 6 feet from where I expect to be hitting and shuffle around it and split step before hitting either a forehand or a backhand. It is just far enough away that I'm having to move my feet to get into position to hit a groundstroke. There's a video of the fuzzy yellow balls guy doing this on youtube.
I also put three target cones out, with two about three feet away from each side and the baseline and the middle one three feet away from the baseline. I then alternative hitting toward each.
When should you serve and volley in doubles? Only when you want to win.