Ideas aren't valuable. Everyone's come up with every idea ever, and tons of interesting ideas pop up all the time. It doesn't matter how great the idea is, how clever or complex or unique or super-fancy-never-been-done-special.
A bad writer is going to do a bad job, and a good writer is going to do a good one. Execution is more important than any special 'ideas'. 1/100 stories I see (being very generous here) posted online are what I consider 'no-criticisms publish-worthy well-written'. Everything else, even if a genuinely enjoyable read or reasonable well-written, is absolutely nothing special. Nothing worth stealing from.
People who steal are most likely bad at what they're doing anyway, so they can't do anything with it. People with any decent skill don't need to steal. Ideas are the easy part. Anyone can come up with some cool ideas. It's developing skill and quality that's the toughy.
If you're really that worried about it though, fanfic or unrelated short stories/writing is a good idea. Anything to develop your personal skill and get feedback on your general writing ability. I've noticed, it usually doesn't matter what it is the person's writing, or what particular excerpt of their stuff you read; their general strengths and flaws are recurring patterns that I can eventually see appear in all of their writing.
edited 16th Feb '11 9:01:41 PM by almyki