When doing step ups with a barbell, how do I minimize the amount of cheating I do by pushing off from the ground at the beginning of the movement?
Firstly, make sure your step-up is relatively high (i.e. don't use the Body for Life aerobics 3-inch step blocks). This maximizes range of motion for the working leg.
Secondly, stand your non-working foot on tip-toes (full stretch) immediately before you start your working leg. This takes your potential energy contribution from your non-working foot to almost zero, thus allowing you to focus yourself into putting effort into the working leg only.
It's kind of tricky to avoid cheating entirely. But I like to use a "stomping" motion with my elevated foot while making sure my supporting heel is the last part to leave the floor. I feel that this minimizes the assistance from the other leg.
Standing on your toes is an interesting idea as well, but probably requires a rather high block or bench to not reduce range of motion too much.