Are there good ways to approach a professor for a postdoc?
The best way to do so is simply to email them. If your research interests align with theirs and your publication record is sufficiently impressive it'll start a conversation at the very least. Don't forget to attach an updated CV. If you're uncomfortable with this (or your publication record is not sufficiently impressive), some avenues to get their attention could be 


Have a mutual professional acquaintance facilitate an introduction.
Look through their publications, find one that you're interested in and begin a discussion through it: how would you improve it? If you have amazing ideas that'll get their attention.


Be advised that funding is a major issue when taking on a postdoc. Some pure math fields do not attract research funding, so make sure that the professor has funding to take you (e.g. their website says they're looking, or they are advertising in mathematics mailing lists).

Good luck!