this is really hard to do.
you can use a prog like A4Proxy or Multiproxy but the trick is in finding a bunch of truly anonymous proxies to keep your real IP from being identified. I haven't come across any truly anonymous proxies.
Some anonymous proxies will spoof your IP when you post up at forums, but if you do a security test at wvw.auditmypc.com they always seem to identify my real IP.