The quick response -- you can't. They do it that way intentionally, because they want to charge more money for people who want a permanent IP address.
Now, what is more useful to you -- how to get around it changing every now and then.
Get a router, such as the Linksys WRT54g, or WRT54gs, which can give you dynamic addresses inside via NAT, while maintaining only one IP externally. One of it's features is called Dynamic DNS. You use a service, like http://www.dyndns.com/ and you can then have a domain name that resolves to an IP address. The dyndns service and the wrt54g talk, and every time the IP address changes, the wrt54g notifies dyndns, and it updates the IP for your domain address.
The best part? Basic service is free from dyndns.
Also, you will need to put a firewall rule (in the wrt54g) to do port translation of the HTTP/https protocols to the specific inside address of your web server. Send me an email if you get that far and get stuck.