I am quite small and have hip problems so when I started riding at about 14, it was not easy for me to mount. Fortunately my instructor had steps and allowed us to use the fence to mount as well.
I found that as I rode and my riding muscles grew nice and strong, mounting became much easier for me. It wasn't really that I became able to mount well, just that my body was more accustomed to working.
In all though the best way to get better at mounting is to just try and do it :) A few push-ups couldn't hurt, though.