This should really be in the "Anonymity" section, not general discussion.
First. There is no way to be 100% anonymous.
The golden rule is simply to be harder to find than those who are looking for you are willing to endure.
Therefore, if you hack someones runescape account, chances are no one gives a shit. If you hack into a government, you better be pretty damn hidden.
The best way is by chaining a different types of proxies, services, and removing any other personal identifying information.
The most often used types of proxies are HTTP, SOCK4, SOCK5. You can look up the differences yourself. It is best to use private proxies ,especially ones where you know your actions aren't being logged.
There are also VPN's which you can connect through. You can look that up as well.
Another method is by using a public network. Drive into a nice neighborhood with your laptop and find a unsecured wireless network, connect from there.
Lastly some people say TOR is a good program, others disagree, at any rate if you are doing something simple it will probably do the job, you can google that as well.
The best way to stay anonymous is to combine a lot of these methods. I would connect to a VPN and through a proxy from there personally. Also like it said it depends on what you are doing, always think about the possible consequences before doing something.
Basically I think as long as your entire is computer is using these methods to connect somewhere, it will work for metasploit as well.