Putting a little extra "stuffing" into your favorite waiter's stocking is a nice gesture.

[edit] Steps

During the course of the holiday season, enjoy a meal at the restaurant, and request that you be seated in this server's section.
Take a simple gift with you for that server. Select this gift based upon how personal your relationship is with him/her. A box of candy is a standard. A book related to a topic in which you know he/she is interested is a little more personal.
Attach a card that will also contain the monetary gift.
The "appropriate" sum is up to you. You want to be generous, but stay within your own budget. A minimum amount to consider is $25.00; there's probably no maximum, but you don't want to embarrass the recipient.
After the check is paid and after you have tipped him for this restaurant visit, present him/her with the holiday gift.
With a smile and a handshake, let him/her know how much you appreciate his/her service and courtesy throughout the year.
Avoid "hanging around" waiting for him/her to open your gift. Be discreet. Remember: he/she is at work.
As you say goodbye, remind him/her not to toss the envelope.