In on at, is there any simple strategy I can follow when I use them?!
If you want a totally honest answer - no .
These are preposition of time, place, direction, movement, etc and, in my experience, drive students crazy even when they are really advanced. Under each category of use there are some "rules" and useful things to help you remember, but a simple strategy? I don't think so. Just hard work and a good memory - as I am finding trying to learn the equivalents in Spanish.