There are some very good recommendations here and I agree with most. I just want to emphasize on "Keep in touch with ex colleagues". It is extremely helpful even if it doesnt seem so at this point. I quit my first and only job 2 yrs back to start my own thing. When I left i left with a smile and maintained contact with all colleagues and bosses. I cannot tell you how much business I have found with the reference from my ex colleagues and bosses. Especially bosses.