I belong to a few.
A few tips.
Fill out your profile.
Be honest.
Don't Expect too much at the start.
Spend some time getting to know the site before you join.
Add comments or suggestions at the end of the surveys if possible.
Take the surveys seriously, They need good data.
Be consistent and timely in doing the surveys.
..and hope you win the draws.

one dayI WILL try one

im signed up with a few of these. the best in my opinion are global test market, web perspectives, and ipsos.
there are a couple companies that take sign ups for paid focus groups to. so far my experience with those has been great. I have done 4 focus groups and made around 500$.
One survery site ive been searching for a registration to is pinecone research. its really hard to get on it... hint hint hint......

Yes I would like to know as well what one works the best and gets you something in return for your time.

Thank you for these links...some were dead but a few were very helpful.