then you'd use a dynamic DNS service like
www.dyndns.com, is this what you're after?
edit: ok, just seen your above post.
You do want a dynamic DNS service. This works by having a client running on a system within your network, whenever your WAN IP changes it updates the dynamic hosts DNS record with your new address.
Register an account at
www.dyndns.com and then get the
update client.
To get individual services accessible from over the internet that are already available on your LAN you will then need to forward the appropriate ports. You'll have to specify what services you want for further help with this.
This is all totally free BTW