Protect your PC
Of course you already know that you must protect your computer from viruses, spyware etc. So you must have good antivirus/antispyware/firewall software. Also I suggest you use Mozilla Firefox with AdBlock Plus and NoScript add-ons. Don't forget to clear all cookies (and if you want all personal data) when you logout from a PTC site. The new versions of chrome and IE 8 Come with built in ‘cloaked surfing, Firefox will not be far behind.

Do not use the same password in every PTC site you join
Some scammers create new PTC sites because they want to "steal" your personal sensitive information. If you do not want to see your accounts hacked do not use the same password in every PTC site.

Sometimes a PTC site may get hacked, and the hacker will then try the same user name and password on various other accounts, emails, gateways, PTC sites.

A good suggestion is to use 2-4 different usernames too. Finally if you join the forum of a site, it is recommended that you use a different email account (and of course a unique password). Forums are being hacked usually easier in comparison with the main site.

Invest only, if you can afford to lose all your money.
This is a basic rule applying not only in PTC, but in every type of investment. When investing, you are taking a risk: you may get your money back plus some earnings, you may lose it all.

Don't invest in new sites. The word "new" does not mean only sites which are only a few weeks old, but also and sites in which you are new member. It is recommended that you avoid investing in new sites and that you be member for a short period (and get paid too) in a site before you invest.

Do not cheat
You will be banned, Don’t do it!

Never use your own money
You never know if the site you have invested your time and money in will turn out to be a scam, so if you’re going to invest, use money you have made from the site or other PTC sites. If you must invest money then don’t put more that $30 -$40 as keep it low so you can get it back fast