here's a list of some dynamic dns services
http://directory.google.com/Top/Comp..._DNS_Services/
it works like this, you run a small client on your machine that detect new dynamic ip address, it then connect to one of those dynamic dns services and update their records with your new ip.
so for example you signed up with one service and registred for savo.dynamichost.org
so every time you get a new ip address a client will connect to their service and tell them that ip address for savo.dynamichost.org is such and such. then you just enter savo.dynamichost.org in flashfxp and every time you run it flashfxp will bind itself to correct ip address.