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[ I am assuming the following scenario -- you have ssh access to a box that you admin, and you want to change firewall rules. You don't want to accidentally filter yourself out. Been there. ]
Along the same lines of the previous recommendation on handling this, here are two more:
You can make the very first rule in your INPUT chain to allow access to port 22 from your source IP. That way, even as you append (and possibly break) later rules in the chain, your first rule should still always allow you access.
While making any changes, you can first back up your current firewall script file to iptables-rules.orig. Next make your changes to the iptables-rules script. Then, before running your updated iptables-rules script, submit an at job that will run in 10 minutes. That at job will simply run the iptables-rules.orig script. If you really bork things good, the at job will save you. If your updated rules are ok, then just cancel the at job.
Hope those ideas are helpful. (To give credit, they're not my own ideas; they are both based on firewall management strategies I read in "Mastering FreeBSD and OpenBSD Security", by Yanek Korff, Paco Hope, Bruce Potter. No, I don't work for O'Reilly.)