But I think, hands down, the best advice I ever got was to never be shy about saying "no" to things. You're more likely to go broke by saying "yes", so make "no" your default response and only switch to "yes" when you can rationally support that decision.
The second-best advice I ever got was the "chandelier rule": when you're being pitched some sort of deal, the bigger the chandelier in the room, the worse the deal is for you. That is, be very, very suspicious if someone is treating you opulently when they're trying to sell you something.