I'd advise new teachers to start teaching before you actually get a job. Begin putting your curriculum together. Decide what subjects or grade levels you'll most likely be teaching - even if you haven't yet been hired - and begin to prepare. With every article you read, think about how to turn it into a lesson - how to bring a particular concept to life for students. Meet with more experienced teachers, raid their files, and build your own before you get a job.
Once you get a job, remember that your first years are a rehearsal for the rest of your career. Develop good curricular habits; be cautious but don't automatically shy away from controversy. Don't be a technician. Create your own curriculum. I think a lot of people, when they begin teaching, start following or trying to find other people's curriculum. Be a creator, not just an instruction-follower. This is not to say that you can't use other people's lessons - there's a lot of good material out there - but see yourself as a producer of curriculum, not just a consumer.
And don't be a Lone Ranger. Teaching can be isolating if you let it. Establish a support group, a study group, a critical friends group, an action group - whatever you want to call it. Just because you may be in a classroom all alone with your students doesn't mean that you should reproduce that isolation outside the classroom. And don't look for support only from people locally. Subscribe to Rethinking Schools, the Rethinking Schools listserv, get the Teaching for Change catalogue (www.teachingforchange.org) and buy everything you can afford. Join the National Coalition of Education Activists (NCEA). Go to conferences. It's important to feel part of a broader critical teaching community and part of a broader movement for social justice.
If you have time, observe master teachers, take notes on everything they say and do and figure out what you can emulate. Make the school your home. Put down roots in the school community - go to students' games or performances; sit next to your kids' parents. Learn all you can about the community your school serves. Call parents, make them your allies. Especially call them to praise their kids and to learn more about them, which a lot of times does not happen at all.
And finally, keep a journal. Figure out a way to distance yourself from the pain. That first year can really be tough so find a way to pull back from that, to be able to think critically about it and to not be swallowed by emotion.