Treasure Trooper offers some of the better long-term membership benefits I've seen. It has multiple prizes of 100 dollars from playing the treasure game and playing a forum game. The treasure game will take you a while to complete, but that is just the icing on the cake! This site has many pros and cons:
Pros:
High paying daily surveys (8 total)
Lots of cash / reward offers
20% and 5% 1st and 2nd level referrals respectively (sorta)
Able to buy unreferred people using pearls
Pearls can be obtained without hurting your income (basically a bonus item)
Cash Tasks (intelligence tasks) that pay between .30 to 3.00 dollars
Cash Tasks always credit when successfully completed (you can fail if you get incorrect answers)
At least 2 long term bonus games you can do for 100 dollar pay outs (dragon and treasure hunt)
Extra trivia questions on the site for even more money
Cons:
Surveys take 20+ minutes to credit
Surveys don't always credit - Says you don't qualify when you're almost done with a survey
Surveys don't credit when it says you've successfully completed
Usually only about 6 of the 8 daily surveys are available
Cash offers don't always credit (same with every other site out there)
Cheesy
Now for the tips:
Referrals or Gift Cards - You can turn plantium coins into pearls or gift cards. Decide what you want! Pearls can be used to get referrals, which is always a nice thing. The catch is most of the time the referrals you get will get little to no income for you, but the POTENTIAL for long term income is there.
Sit in the chat room - When you are doing ANY cash offer sit in the chat room with a registered name. In order to get accepted for any cash offer, even for one dragon scale, you will get a link for an EXTRA pearl bonus game. Statistically you will be able to buy 2 referrals for every 13 cash offers you complete.
Pearl Necklace - Always use these during the afternoon. This is because most of the offers on the site are intended for the U.S. or Canada. It won't do you much good getting a bunch of referrals from Europe who are unable to complete offers.
Clear your cookies - After you do 3-4 cash offers it is a good idea to clear your cookies. The idea behind this is that the same company hosts a lot of similar offers and the cookies might get confused with an offer that was already completed.
Watch what you download - An offer to install something for a dollar might be tempting, but it might be so loaded with spyware that you will spend hours undoing the download. I recommend scanning every file with http://virusscan.jotti.org/en before you install anything. Even with that I would use an older, crappy computer to complete this offer instead of my main.
Be sure to read and follow all the general safety/tips outlined on this site.
