Oh boy... I could write a book on this!
With Zefra, my little one all I have to do is look at her and she is ready to go... LMAO. She was born to work and is always waiting to do something
. With her I use nothing, treats, ball, tug, etc... anything really that I have around with me. I sometimes use her leash if I do an impromptu training session as a tug.
Now, with Stark..... oh boy did he teach me a lot of things!
He wouldn't work for a tug, a ball, food, any toy that I purchased or made, etc... he wouldn't even work for me no matter HOW MUCH I tried.
What I ended up doing that worked for us was to back tie him and tease him with the tug or ball until he was super frustrated, then put him away. I did this for a good while before I let him "win" the reward.
Finally after a looooong time, when he saw that reward come out, he would go nuts for it! I finally started asking for little things (sits, downs, heeling, etc.) and then would reward with the reward I had frustrated him with for so long.
What I also do is play with him without anything on me. I push him back, let him jump on me, allow mouthing a bit, and I really, really push him around (he LOVES THAT!). He gets so excited to play rough like that with me that the behaviour is the reward in itself.
Also, using the tug to kinda play "smack the puppy" with. I would take the tug and kinda hit his bumm or side. Drag it over his face really fast without him being able to get it. I would do this for awhile before I worked with him and then would use that tug as the reward and if I felt like I was loosing him at all (drive/enthusiasm/etc) I would start over again and kinda smack him around with it... lol.
With Zefra, to get her REALLY ampt up (which I don't really do because with her I have to work on CAPPING that drive a bit, working through the drive instead of going nutso and not listening to anything) is take the ball (always on a string) and wind it around me, through my legs, etc., keeping it away from her. Then I bring it into me, ask for my command and then release her to get her reward. Really though, with her, I can't do too much drive building before any real training because otherwise she is just too over the top.