I've been spending a lot of time filling crap out on the internet for money. And since I'm a full disclosure kind of person, I'm going to tell you where I did it and how to do it like me cause it's great! I bet you're sold now. I thought these sites were such a great idea that I've even started my own!
Anyway. For the basics check out Money Dummy's guide - the importance of clearing cookies, multiple email addresses, etc etc. In general, what I did was to go through different sites, fill out the easiest offers and move on to the next site. I was bemusedly pleased to find out that I won a contest on one site by being the person to make the most money in the previous 72 hours - an accident of timing, but one that got me an extra $15 anyway. Woot.
Have a lot of credit card numbers. I have a Stockback MBNA card, which is really the best kind of card to have for doing these kinds of things since MBNA has a program called ShopSafe which will generate one time use credit card numbers which are only good for a few months and you can set it to have as low as a $1 limit (so if you forget to cancel a trial or something, it'll simply bounce. See if your credit card issuer has a similar program - I know I've seen others but I don't know what issuers have them specifically.
Definitely use two different browsers - I use Firefox and IE - so that you can clear the cookies repeatedly in one, while still having your email open and what not in the other. (I have a link to download Firefox on the sidebar - it's a neat little beastie.) Make sure to block pop-ups.
Check DealBarbie's forums - people will often pay you to sign up under them, or swap referrals with you. However, I think for the sites that DealBarbie has listed, where you get money from DealBarbie directly for signing up, you need to sign up with her referral.
Of course, I think my own is the most wonderful. CashDuck's philosophy is that offers shouldn't cost you more than you get back - I aim for any spendy offers to pay at least twice what you spent, and I have a LOT of freebies. Also, nothing costs more than ten dollars. There's also a cool prizes program where you earn feathers (aka points) in addition to money and can get gift cards and what not for your feathers.Go look at CashDuck! Yay!
Here's what I think of the other sites:
DealBarbie: This was where I started. It seems to be the best-run site and certainly has the most offers. Cashout minimum is $20 which you can get pretty easily by filling out a bunch of the $1 surveys. Clearing cookies is key here. Also, check out the forums if you have any questions - they're very active. There are also a lot of $2-3 offers for information on various schools, health insurance, car warranties, home loans, etc.. just make sure not to use your real phone number. But you can definitely make cashout (and much more) simply on the hundreds of free offers.
MyHotCash, RockHardCash, HoorayMoney, InstaCashVault, PocketCashGPT, MadCash4U: Very similar sites to DealBarbie. There are standard software packages for the sites and once you have done a few you will be able to quickly tell them apart. There are not quite as many offers as DealBarbie but the sites are not as old, so after you complete your first round of offers, check back in a few weeks. I won the $15 on RockHardCash. I recommend RockHardCash, MyHotCash, and MadCash4U.
InstantProfitz, FusionCash: Similar packages. They are entirely cookie-based - the best way I found to do this without losing my sanity was to do all of my regular web browsing in Firefox, and all of the offers in IE so I could repeatedly clear IE's cookies. There are a decent number of free offers, but I kind of got fed up at the cookie-ness since you have to keep better track of which offers you completed, so I did a couple offers and cashed out. If you have more patience than I, InstantProfitz seems to have higher payouts, but MoneyDummy says they have a worse record of actually delivering the payout for the offer.
DontCostAnything, AnyGift4Points: These sites are a different software package entirely - they are based on "offers" which is kind of a unit. You need to do one offer before you can start accumulating money. There are fractional offers - generally the more of an "offer" it is, the more likely that it costs money.(Note: if you sign up for one of these, I'll split the bonus I get for a referral with you. Send me an email if you do sign up.) DontCostAnything and AnyGift4Points have systems where you fulfill a certain number of offers and get a certain prize (ie do 3 offers after the first one, get $130.)
Prizebook: I did 3 tickle offers and the entertainment.com offer on Prizebook, and it's a really beautifully designed site. They credit in dollars and cents. Don't be tempted by all the pretty things you can buy with your prize money, just get it by Paypal and buy it on sale!