To continue with Jay's excellent advice on disrupting spinal structure, if possible, an uppercut-like strike to the chin could help. I say 'uppercut-
like' because I don't want you knocking out your training partner!

However, a sharp slap with the back of the hand could suffice.
Destabilizing the 'base':
The knees...ah, the knees. Now, don't break the knees! Nobody will train with you anymore if you break someone's knee. However, if you can somehow destabilze the knee, that will generally cause most people to bend one way or another to catch their balance. A trained person will just drop their weight to regain control. You can still use that, though it's a bit harder.
Light foot sweeps. I don't mean to take them down with the footsweep alone. Just enough to make them shuffle, which you can capitalize on with a strike or just move straight in for a takedown.
Cthulhu
PS - a Louisville Slugger to the base of the skull will take most folks down, too
