My first time sparring I caught a 15 year veteran in the game with an uppercut, he was going ridiculously easy and his love taps would do some damage but I watched his movements and stepped forward with a rear uppercut and caught him flush, probably the greatest punch I ever landed on someone that experienced (He was born and raised in the gym).
I like to dip forward when an opponent rushes(Almost like throwing a body jab but instead use the movement to set up a rear uppercut) and bring the rear uppercut straight into my opponent if they are swinging aggressively and stepping straight forward.
Also I like to just wing them from time to time kinda of like Alexander Gustaffson, but for the most part that punch has to be set up and timed correctly- if you just throw a rear uppercut as a base or frequently you might end up like Schaub when he fought big Nog, horrible uppercut that got him Tko'd.
I love body uppercuts too, my favorite technique when I get in close and we are engaged in short range- is just a pair of sharp, fast body uppercuts from the same hand and if my opponent folds/flinches or doesn't move I add in the other arm for the combo. After about 2-3 good punches on the inside I usually start angling in range as much as possible trying to corner said opponent.