Apart from the obvious and pay up front etc.
Don't knock back the opportunity to assist an engineer in the hangar. You'll learn heaps.
Don't think that because it is expensive it is the best. Also, don't always go for the cheapest price either.
Don't be a UK is best flying snob. The USA isn't all sunny and flat. Neither is Australia or Africa. If a year or so in those countries means you get to do more than "just instruct" or "burn holes in the air" then it's a good thing. Doing night freight in the US in a chieftain or Caravan is better than circuits in a C152. Experience counts.
Be prepared to travel to get a job. Be prepared to fly a "lowly" turboprop or even piston.
Be prepared to do ops. You'll learn something.
Accept that there'll be hiccoughs and low days. Realise it and move on.
Accept that no one owes you a job. No one forced you to fly, no school has to find you a job.
Understand quite clearly that there are more pilots than jobs in the world.
Understand that you are supposed to have fun and enjoy it.