Well, my experience is that the first one is almost always rubbish! But, that said, with thicker ones, I've found you need to cook them in a mixture of oil and butter (oil for heat, butter for flavor). Also, don't let it get too hot or your outside will burn and the inside be raw. I find a medium heat is best. I also keep them warm in the oven which, if they are a tad underdone in the middle, will help that cook.
I check the side cooking (just peek under there!) to make sure they aren't getting overdone. And if they are very thick, I might make a small cut in the center to see if they are underdone.
Hopefully someone else will have some clearer tips but I find it is a bit of trial and error!
Ah, cross-posted with Mrs Pink... the bubbles are a good indicator. Also, a dry edge.