Load balancing in this scenario will probably require quite a bit more than just adding another static route. However, there are two key determinants: what link protocol is running on the T1 link and does your DSL link use the same public IP(s) for you as your T1 link.
I am assuming your T1 is Cisco HDLC or PPP, but if it is frame relay you will need to add a mechanism to detect when your PVC goes down and the LMI stays up (hint, you can use whatever technique you use to detect network down on the DSL link, see next below).
I am also assuming you have a standard DSL and your DSL "modem" is doing NAT (implied by your original statement of the problem, but not explicitly called out).
In this case, you have several issues to deal with before you even consider load balancing. The first is detecting when one of your links is down so that you do not continue sending outbound traffic down a dead line. On a point-to-point T1, this is trivial, as the interface will go down any time the link has problems. On the DSL link (and sometimes your T1 if frame relay), the router interface will stay up even when the link is down. This makes for bad surfing...
You might be able to get Verizon to run a routing protocol with you on your T1, but this alternative is probably not available on your DSL line. For your DSL line, you have three choices - you can ignore the problem and lose communications for whatever services you allocate to use the DSL line, you can forego load balancing and only use the DSL line when the T1 fails (and hope it is functional when you need it), or you can use ping based routing (search for the "RTR" acronym on CCO).
Once you can reliably detect which links are up, you can use policy routing to select which interface is the preferred route for which traffic. You just need to make sure that all traffic on a particular connection (such as a TCP connection or sequence of UDP transactions) will always use the same link, otherwise the other end will see two different IP addresses for your box and fail to communicate.
Finally, you will need to make sure that the correct NAT is done regardless of path, so that the NAT is correct for the path taken. This should not be a problem for you, as you state that the router is not doing NAT at all. Cisco routers have a problem with NAT when the same address pair needs a different NAT depending upon which outside interface is used.
Good luck and have fun!
Vincent C Jones
www.networkingunlimited.com
