or the paganini rachmaninov.
The thing doesn't necessarily have to sound EXACTLY the same. Rachmaninov skillfully manages to imply to the audience the overall theme, without the actual theme surfacing.
and the infamous 18th variation, you can't even hear the theme!!! its actually a retrograde of the original theme.
some Key techniques of variations that i can think off the top of my head:
stretch out the theme (in terms of time value)
change rhythm
change mode (maj -> min etc)
change key
inversion
retrograde
then if you wanna be a bit tricky, you could possibly superimpose a couple of the techniques.