I use whatever works. Cut up raisins, slivers of cheese, kibble when I have it, bits of leftover meat. I also switch from treats to tug of war for routine cues like sit, down, heel, stay, kill. For a dog that enjoys tug, having him do a sit for a couple of seconds, then giving the "kill" cue brings the new cue on the road. Tug a bit, then "give", a cue, then, kill again. I start with making the dog wait just a couple of seconds, then gradually have him hold the sit, down, stay longer and longer. And praise, of course.