The way I do it is to drag out a construction line from the first circle to the second circle. If you try and do it by eye from the first circle when you drag over the second circle you should get a "tan/tan" point to snap the construction line to. Release the mouse then go back and draw the line using the construction line snapping points. I find sometimes after getting the "tan" snap on the second circle you sometimes have to drag the mouse ever so slightly to get the "tan/tan" snap.

Hope this helps.

3DKiwi

Update: The way the other's describes works as well. There is no need to create a construction line. The trick is to roughly guess where the first tangent will be.