Gentle stretching. You can ice it, which would probably help a lot, but I didn't and I could run (with a slight limp and very slowly) around 3 to 4 days later. After a week, pain was pretty much gone. The way to do it is walk fast or slowly run about a half mile (to warm the muscle) then do the groin stretch. You can also take ibuprofen or Tylenol to help with the pain, but be careful not to take NSAIDs before a run.
Here's a good video (I'd say do 3 sets and hold for 30 seconds):http://www.ehow.com/video_4460173_do-groin-stretches.html
The butterfly stretch is also very good for that. There's a video on the site above for it.
Doing the high mileage (I think I went from 25-28 one week to 37 the next week) too fast is what did it for me. Be careful after the pain is gone. The muscle may still be damaged and it's probably a good idea to keep stretching and/or do some strength building exercises later, maybe using the circuit machines.
I have to say I probably ran through the pain during the week after the pull, with each day getting progressively better. Pace was way off, but I made sure to go slow. A week after, there was a tiny bit of pain, but I could run normal pace, and didn't try to go faster until about a week later.