Mobile monetization is both a science and an art.
I'd say the biggest tip is to try to see where the market is going, instead of playing catch-up with where it is.
A perfect example: Scoreloop went with the virtual currency route very early, knowing that freemium would become more widely adopted in the future. It paid off. We're currently the leading cross-platform social environment / freemium driver for mobile games.
The software world moves very fast. Something I think may end up becoming interesting in the future is this "pay later" mechanism (Facebook is tinkering with it). This may end up becoming the norm for micro-transactions in a few years time. Definitely something that serious players should consider looking at.
Here's a link:http://www.sfgate.com/cgi-bin/article.c ... 010-11.DTL
It's mostly play-it-by-ear. The biggest advice I can give is to stay in constant contact with as many developers as possible and try to get a feel for where the industry is headed...and then get there first.