This is very easy to do! All you have to do is give out the email address username+insertanythingyouwanthere@gmail.com whenever you sign up for something on the internet that you think may not be safe.
In the event that you did give your email to a spammer, don't worry! Just create a filter, and make everything that is sent to username+whateveryouputhere@gmail.com go to the trash. That way, the spam that you might get will automatically be deleted!