A technique I've employed is holding a code review of my own code. There's an awful lot you can say about someone else's code and have it come off as self-criticism.

...at first I had it coded like this {show nasty code} and decided it'd be more efficient/pretty/maintainable like this {show good code}...

and then go on to explain why it was more maintainable/pretty/efficient.

And you don't really have to beat it to death either, and certainly not drop any snide remarks about the bad stuff you've seen in their code. Just make sure that the bad stuff you *do* show them is your own: they may recognize their own code style.