If you want to bypass registrations and such then you are on your own. Dodging limits per hour we can do.
There are three primary things done.
1) Your IP. Change this. A simple proxy (such as you might find on
http://www.samair.ru/proxy/ ) has a function called xforwardedfor so they might not work. VPN and big boy proxies (possibly something like
http://hidemyass.com/ ) are a different matter. You are not the only person with this idea though so it can be tricky as someone else might have tried in the hour cooldown or something. If you have parents, friends, work and such computers you can play with there are ways to do easy VPN (I think the tearmviewer stuff has the option still, if not tunnelling over SSH is not so bad
http://www.revsys.com/writings/quicktips/ssh-tunnel.html
2) Cookies. Obvious really. Clear the last cookies and cache form the last however long you need, or better just use porn mode and close the porn mode between attempts.
3) Other browser storage. This would be things like flash cookies/lsos.
I am not aware of any download sites taking more security focused approaches and softer approaches/fingerprinting (
http://mybrowserinfo.com/ ), or worse, but it is easy enough to do if they wanted.