I'm with all of the people suggesting you ask them what motivates them. You can set up an anonymous survey (a la SurveyMonkey or using Google Docs) where they can provide their feedback if you want it en masse (and not to seam like an interrogation).
I would suggest asking questions, not just giving them the "what motivates you" question. Things like:
* What kind of office benefits are you most interested in (massages, breakfast/lunch/dinner, gym access, childcare, etc)
* What kind of workspace benefits are you most interested in (access to more closed-door offices for conferences/meetings/private conversations, 2+ monitors, etc)
* What kind of flexibility benefits are you most interested in (remote access, work from home days, flexible in-office hours, FedEx/20% time projects, etc)
* If offered, which of the following rewards would interest you the most (project-based bonuses, quarterly bonuses, time off, more office/workspace/flexibility benefits, etc)
* What type of review system are you most interested in (peer review, quarterly goals, management review, etc)
* How satisfied are you with the current review system, current benefits, etc
* What would you improve about the current review system, current benefits, etc
Most of those don't have one-off answers, they may be a ranking type thing where people are MOST motivated by something but would also be interested in something else.
As a manager of a team, it benefits you (or whoever their manager is) to motivate each team member individually, as their interests and motivators will NOT all be the same. The survey or data you gather will tell you how satisfied people are and give you an idea of some commonalities among your team members (like someone said, if the massages are totally useless, get rid of them).