1) Don't just trust the status of a member, anyone can write 500 posts and be a hero member in a short period. This just means that he invested some time on his membership. He can always decide that a profitable scam will be a good payment for his time.
2) Never deal with anyone without doing a search in the forum with his nickname, in order to check if he has scam accusations.
3) Ask for his coin address and always search it on blockchain.info or www.cryptocoinexplorer.com to verify if his address has the amounts he is claiming.
4) Be very suspicious about proposals well under market price or if he accepts your proposal under market price with no negotiation.
5) Look for the reputation of the member (his successful trades, in the form of vouches from the other party on the deal).
6) Even if he has some reputation, never send all the money in one transfer: send one first very small tranche (0.1 btc might be a good start) and wait for his payment. Send a second little higher tranche and wait again, etc. Small increments, because he might decide to pay you the first and second, to scam a bigger tranche. Always suspect if he doesn't accept to trade in small tranches.
7) If he has no reputation (including most cases when he is a newbie or a Jr.), even small tranches are a risk. Better use an escrow system, like the one of bitmit.net, or a trusted member, or you might need to send over 10 very small tranches. It can be annoying, but loosing your money will be more.
8] For big amounts always use an escrow system, unless the member has an excellent reputation.

Since this thread is a sticky, I decided to post this here. Any moderator or member feel free to move, copy or improve these rules at will.