I keep these on my calendar but I have a RTM list specifically for sending cards or gifts. Occasions that just require a card have a task reminder set a week prior to the actual day, gifts two weeks prior.

Yeah, I just found out too that rtm is perfect as a birthday calendar!
I used to have an "analog" one, hanging on the wall. But then i always had to look a while ahead to see what's coming up.
Now I've added birthdays in RTM as tasks repeating yearly. I also fill in the year the person was born.
So good so far, but that is not so different from the "analog" one.

So I've added extra reminders for (almost) each birthday.
When I believe that I want to send a card - I set an extra, yearly repeating task about 10 days before the birthday.
When I believe that I want to also send a gift I set a reminder 2-4 weeks before, depending on whether the person lives in the same town as I do or if he/she lives in another country etc. It also depends on my guess on how difficult it will be to find the perfect gift... (not so difficult for friends' small children but very difficult for my brother for example)

There are also a bunch of people who I only need to call on their birthday and they don't get an extra reminder. It is enough to see that on the very day.

I think that this really fits in to "clear your mind" (the GTD way) since I never have to look at what's coming up but instead I am sure that my system reminds on the right time.

I love RTM!

I use a calendar too, but I use a smart list to send tasks to my calendar, so I any task I want to show up in the subscribed calendar on Google Calendar, I just tag with #gcal and it shows up there. It's kind of a marriage between keeping all those dates in RTM (which I don't like, since I'm a purist, and things like birthdays aren't tasks that can be checked off =D ) and keeping things on my calendar.

I use the 'Sleeper tag' method. Takes a bit of experimentation to wrap your mind around it but it is pretty cool. Read more here: http://blog.rememberthemilk.com/2009/04/tips-tricks-tuesday-using-sleeper-tags/

I'm probably being a bit thick but I'm very new to RTM. I've set up a birthday list and entered some birthdays. How do I set a reminder separately?

deegreigdunn,
It's not currently possible to set a reminder for an individual task. However, you can choose when you will receive reminders for your tasks by using the Reminders tab of the Settings screen.

Hope that helps!