Are you planning on load sharing at all over the two links, or not? It sounds like you're not. Are both links coming in to one router, or two? I'll assume two, and rework things if you reply with one. :-)
There are a couple of different things you can do here. You have to deal with inbound traffic, and outbound traffic.
For outbound traffic, you can just advertise a default towards both routers all the time, but make the metric on the router with the backup link such that it will only be used if the default off the primary router is gone. One sort of easy way to do this is to accept just a default, and set its local preference on the primary router high. Accept just a default on the secondary router, as well, and set its local preference to something lower. Since you'll be running iBGP between the routers, the backup router won't ever prefer the locally generated default unless the primary link has failed. This would look something like this:
router1
!
route-map setlocalpref permit 10
set local-preference 110
!
router bgp 65000
neighbor x.x.x.x remote-as
neighbor x.x.x.x route-map setlocalpref in
neighbor y.y.y.y remote-as 65000
router2
!
router bgp 65000
neighbor x.x.x.x remote-as
neighbor y.y.y.y remote-as 65000
Note this will work, as well, if both ISPs are connected to one router, just using the route map on one of the two neighbor statements on the same router. Now, if you generate a default route from both routers into your IGP, things should work, outbound, as you are expecting. The inbound side has a few more options.
You can advertise your routes to the backup ISP with really long as path prepends, and that might work--but only if both ISPs are advertising the same prefixes you're advertising to them. If the primary ISP is aggregating your address space, then adding to the as path towards the backup isn't going to help.
You can also often advertise your routes with a community that the receiving ISP will use to set their local preference. Thus, you could find out what communities your backup ISP will listen to, and advertise a community that causes them to set their local preference on routes you advertise to 50 or so, which would make them prefer the routes through your primary provider. Again, this won't work if your primary provider is aggregating your address space.
Finally, you could use conditional advertisement to advertise routes to your backup ISP only when the primary ISP is down (or rather, your link to the primary ISP is down). For conditional advertisement, see:
http://www.cisco.com/en/US/tech/tk365/tk80/technologies_configuration_example09186a0080094309.shtml
Russ.W
