A rather nice exercise to warm up, butterfly in the service square.
There must necessarily be two,
1) I send the ball diagonally
2) my partner sends it to me long line,
3) I go get it and send it back to the other diagonal,
4) my partner sends it back to me long line...
AND we make it last as long as possible.
In short, we do the butterfly.
This same exercise exists on the entire length of the court but it is a little more difficult to perform, and requires a slightly higher level of play, on the other hand in the service square it is relatively simple and completely achievable.
Video demonstration
Download this sheet in PDF format