I had a tournament last week, and there were no warm up courts available when I arrived. So went to the wall, and warmed up a little bit of groundstrokes, both top spin and slice, volleys, and after a little bit, I started warming up my smash. tossed the ball like if i were serving, directing the ball to the ground and towards the wall, so I can hit another smash. Contacts where quite good, and my smash and shoulders were nicely warmed up. The thing was that it helped my serve a big deal. I was hitting loosely. You could try it. I am a big advocate of practicing at the wall, but rarely do smashes, now I will always do them, specially before a serve practice. That said, do make some warm up exercises before you even hit the wall.