The best wording to politely nudge the editor? Any tips on polite yet resolute phrasing for an e-mail to the managing editor of a journal when your article is under review for far too long and it would be nice to give the process a nudge?

Thanks in advance; specific advice for math journals appreciated.
First, always be polite to the editor. Often academic editors are volunteers. Even if they are not, being polite is always more helpful than being not polite. 

Second, journals often have trouble getting enough reviewers. I have emailed journals and asked if they need more reviewer suggestions. This nudges the journal to think about your submission. And, can sometime help them address the delay in review.