What to do if I'm afraid that my idea will get stolen during review process? I have an idea that I would like to publish. However, the idea is so simple that basically everyone in the field could reproduce the experiment that shows its benefit within a day or so.

What can I do to ensure that my idea is not stolen during the review process by a reviewer, for example?

If the reviewer holds back my paper and publishes it himself somewhere else, is there something I can do to prevent this?
Assuming the journals you publish in allow it, you can submit your work to a preprint archive such as ArXiv.org. You can also put it up on a personal website.

Even if someone tries to scoop you, if you work is ever published, the date it was submitted for review should appear in the final article, giving your work precedence on any other. As for your concerns about reviewers stealing your work, there is not much else you can do besides the above. The peer review system is based on good faith, and hopefully you end up with ethical reviewers.