Eraina - I do the PS2/DS or DVD for each assessment, but buy them on EBay - waaay cheaper! 
Impthing - if we are really struggling, I offer to come back to it later, but he is not allowed to watch a DVD, play a game, meet with friends etc., (yes he has friends now lol!) until the exercises are done. Usually, he comes back within 5 minutes! We do sometimes resort to treats, but I try to keep it to a minimum (we all have off days).
Our local Centre have charts like Spiderman, Simpson to keep them motivated. Ask when you next go.