How do I keep a good idea secret if I have to market research?

If you are doing a research about a particular idea, I don't think that will expose your thoughts, but you also need to realize there are a few people with the them idea, whoever run with it first, own it. Do your research with little giveaways and then give your idea live. :)

If your idea is still a secret try to get it patented. Know that if your idea is to serve a need (if there is demand for it) someone might be already thinking about it. Do your research with minimum exposure.

I think these days it is hard to patent it as there are many ways to go around and copy the idea anyway.

It is not likely that among the people you will survey someone will copy it. It takes some times to prepare and copy an idea. Just do your research and then launch it straight away (if the research findings are positive of course ;)

Write down everything about it and make sure that you date it. Then put the idea and write stuff you can about it and date it make copies of it (put it in a file cabinet or someplace) put the original one that you wrote in an envelope, seal it, make it out to yourself(to be mailed to you.) Take it to the post office and tell them to postmark it and drop it in the mail slot, to be mailed to yourself. When you receive it it the mail at your house, DO NOT OPEN IT! Make sure you put it up in a safe place. This is what they call a poor man's patent. And it's legit if nobody else has documentation of it dated after yours. If you have to go to court over it for some reason, all you have to do is take the sealed letter to court with you and let them open it. Maybe you should mail more than one copy of it to yourself at the same time, just to have them. In seperate envelopes of course.