The difference between a good tip and a great one costs little but means a lot to the recipient (as any former waitress knows). If you're deciding between money and a present, research shows that most personal trainers prefer cash, but presents are always welcome.

During the holidays, you tip the people who perform services for you year-round, who are there for you when you need them and whom you have a valuable relationship with. Hopefully you have a valuable relationship with your personal trainer.

And, tips vary for trainers... a lot of which depends on your relationship with them. For instance, you don't have to give as much to a new personal trainer because you haven't forged a relationship with them yet.

The average tip: One hours session.
The most generous tip I've seen at one of my husband's facilities: One family (you know who you are) gave a trainer a tip of $4,000 to pay off the last of her student loans! - It was an amazing act of generosity.

For more tipping information, on other service people in your life, check these great sites out:
The Tipping Pointer - [New York Magazine]
Gawker's Holiday Tipping Guide - [Gawker.com]
The Ultimate Holiday Tipping Guide - [CNN Money.com]