ssusu makes a great point about keeping ingredients cold. I went to France on a cooking vacation and got a major lesson in croissant making. Laborious as all get out, but so satisfying. I'd say get some French yeast, for one thing - World Market has it sometimes - and follow a recipe that measures ingredients by weight. Also, take your time and keep it neat. It's important that the rectangles you roll out are even. Use unsalted butter. One mistake I made a couple of times was using chocolate that had too much going on. You want a semi-sweet chocolate that's mellow instead of acidy. Don't get all wound up in the "I only like the 70%" mentality. As hard is it is to make croissants, you don't want them to taste like medicine. Guittard chocolate chips, for instance, taste delicious in chocolate croissants, but stay away from products like Scharffenberger.
The "Baking with Julia" book has a recipe that works.
Oh, yeah. The egg wash, which you want for that laquer-y look and shattering bite: paint it on twice, letting it dry in between applications--15 minutes or so.
The technique for making croissants, like puff pastry, is called "lamination". Easy to see why. I like a marble rolling pin, too. Keeps the dough cold and takes a little of the work out of what is a major job. I mean, you roll and you roll. It's a great upper body workout. There's nothing quite as satisfying as pulling your own croissants out of the oven.