I have done a bunch of embedded development in the past. Embedded.com was a good resource. There's a lot to know so it is difficult to be really concise. Get to know lower level languages if you don't already (c, bit operations). Know how to disassemble code and read it. Monkey around with an arduino, then beagleboard. There are a few good books - anything by jack ganssle (sp?) is usually a good start.