The best answer to this is to use something called Border Gateway Protocol or BGP along with two providers that will support this with one another. Essentially, you would have two separtae providers that would provide you with bandwidth, but you would only have one range of IP addresses—which are used by both providers to send you traffic.
The challenge with this is that it costs to do this. Each month for the bandwidth and for the extra charges to do BGP. Plus the routers are expensive (the 2811′s don’t do BGP as far as I know)
Another thing you can do is to bring the bandwidth in and and create a second set of servers to preform any services you provide on the second ISP’s network. This will require either Round Robin DNS or a application layer load balancer, for example one I’ve used is a Web Server Director Pro from Radware.
In Round Robin DNS you essentially create a second set of A record DNS entries for all the hosts that are provideing services on the internet. For example, www.yourdomain.com might have two A records… one on one ISP and one on another ISP’s IP address space. DNS by default “takes turns” handing this out. If you should take this route, be sure and set the TTL very low (2-5 minutes) on these records so that if a ISP link goes down you minimize the amount of time a “unoperational” address is cached on DNS servers on the internet.
The other possibility I mentioned was a application layer load balancer, like a Web Server Director Pro. This is probably the better solution because you can do more than just balance the load. You can detect failures of a application or server as well as shape traffic– for example if one your pipes are smaller than the other one. Additionally, it allows you to scale your servers horizontally to 2, 3, 4, or more servers providing the same services, much like the “big boys”.
The challenges you will have is that you will have to chose where your client internet traffic comes from without BGP since the response back to a client request will be directed back to the ISP it comes from.