Don't only join the best sites. Join as many as you can, be an active member for 2 or 3 weeks, look and compare your earnings, and only then select the ones you find best and want to keep (membership is always free, and the deletion of an account is simple)
Don't always go for the low payouts and dismiss the higher ones. Usually low payout also means low PTC and email value In my opinion, if you want to set yourself one maximum payout amount, it should be around 50$
You can join high payout sites (125 $ and more), as they will often offer high value emails and PTCs.But be aware that if the site isn't serious, it might stop working/be deleted before you manage to reach the payout
Build downlines!
Dont worry where you are based - most of these sites are based in the US, but will accept international members (remember I'm in the UK and have had no problems)
Don't forget that not only do you earn a percentage of your direct (the ones who signed up using your referral link) downline's earnings, but also a percentage of their downline's earnings, and often even their downline's downline's earnings.
All those sites pay you for reading and clicking advertisments..don't forget to use some of your earnings to do some advertising yourself (for example your referral links)

Use the same email account for all the sites you join (or else you'd get lost jumping from one email to another and remembering which is which), and if possible not your main email address (Lots of emails, which might crowd your inbox. For example, having joined those sites, I am getting over 300 emails per day).
Create either a paypal, stormpay or e-gold account or all three - it's free. Those are the main electronic payment systems they use. Most of them also send cheques and a lot offer even other payment methods too, such as esmart pay, Moneybookers (see my e-money page for details)

It's free! Always! So you have nothing to lose (besides the time spent reading emails and clicking on links of course) and a lot to win.