I haven't been scammed yet. But of course my first few trades and transactions were only with people who had tons of posts on the forums or ran a business (like CoinPal) or had a super good OTC rating, and I always sent first and started out with small transactions. Now I have a bit of a rep, and I'll do small transactions with new people (provided they send first), and larger transactions with well established people.
I feel like it's pretty easy to not get scammed if you structure how you go about trading in a reasonable way. Yeah, if you do a 400 btc transaction with a guy you've never seen before and you send first, there's a really good chance you'll get scammed. But that's freaking stupid. How have you been scammed, and is there anything about how you've been trading that you think might lend to scammers seeking you out? Like always sending first, or trading large amounts with new people over Paypal, or something like that..?? Use your head, and plan ahead of time if you need cash for bitcoins, so that you don't have just jump on the first buyer you see. Use not just the OTC web of trust, but your own brain's web of trust. Does the person have a lot of posts and arguments in the econ forum about why Bitcoins are a good currency, and his account was created a year ago? Even if I haven't traded with him, that makes him seem more legit. Does he have a high OTC rating and can sign something form that account's key to prove it's him? Also good.
Or does he just have one post saying he wants to buy bitcoins with Paypal? There's good ways to reasonably protect yourself, even in an anonymous environment like this. It takes TIME to build a reputation, and TIME is something fly by night scammers don't want to spend. So look for people who've been around, and the people that THEY recommend. Build your own "web of trust" via this community and other good places like it.
Of course there's ways to protect oneself, I've only had one person get one over on me from a small transaction early on. However, I see a lot of people try. And then there's things like on BiddingPond where you have non-payers. They just say, "Oh, I forgot that I don't have any Bitcoins." I'm just saying people need to take your advice and use caution, because there are a lot of people out there right now trying to get Bitcoins in any way they can.