I think I'm missing a little bit in the description. You mean your internal network is DHCP'd, or your provider is changing your IP address?
If you just want to make sure you have SSH access to your machine, you can use an authentication key instead of a password, and only authorize your remote machine to have access.
I also install denyhost, which you can configure so that if someone tried to access your machine and failed the password three times your system bans the IP. It can also download a list of blacklisted IP's from other denyhost systems, and you can configure if the ban is permanent or how temporary you want it.
EDIT re-re-reading, I think I see your problem...you have a home system to access, you want it to allow from a static IP, but your remote IP keeps changing so you can't set one specific IP to allow your connection from, yes?
In that case the above things I outlined (key authentication, change ports against an automated script, denyhost) should be more than enough in your situation.
For security through obscurity (it only helps against automated scanning, really) you can change the default port to something else at the firewall/router.
If you're saying your system is DHCP'd in your network, you need to configure it to have a static IP and forward the port to that IP.