The Twitter thing is a good, practical way to help, even though it’s of limited use.
A more useful thing you can do, if you know about computers, is to set up a proxy server so Iranians can bypass their government’s internet blackout.
This blog has instructions (and make sure you read some other blog entries on there that detail how to do it for Macs as well as additional safeguard you should take). Contact the guy running the blog and he’ll get your proxy to Iranians.
For my part, I couldn’t figure out how to do this. So another way to help would be to contact me and explain how the hell to do the following stuff on a Squid config file.