I found this article to be full of helpful info about gurus:
http://www.ncf.net/guru.htm
Here's one interesting comment he makes:
Once someone has found their Tsa-wa i Lama does this mean they should not attend teachings or initiations given by other Lamas?
No, of course not, but a little care should be taken. Each tradition of Buddhism, such as Zen orTheravadin, and each school ofTibetan Buddhism has a different way of presenting things. If you listen to a great variety it is easy to confuse issues without realising you are doing so. It is like a paint box! The red is a nice colour, and those two greens are both clear and bright, and the yellow and that rich purple - all are fine colours but if you mix them all together you get a muddy brown! It is better to stay with teachers of the same lineage as far as you can so that your mind does not become muddy brown! However, a little of one colour added to another can be good. Ask your Refuge Lama or your Tsa-wa i Lama for advice.

