Ingress Traffic

Many large IP transit networks support BGP community features including the ability to identify customer-advertised routes as backup routes, which then receive a worse/lower local-preference than even routes the transit network learns from its peers.

This is easier to explain with an illustration. What you must do is ask the ISP you wish to serve in the backup role if they support such a BGP community feature. If so, your job is easy; simply configure your router to add that community to the BGP announcement sent to that ISP.

If you're not sure what to ask your backup ISP, just send them to this thread. They should understand immediately when they see this diagram.

Egress Traffic

Controlling egress traffic is easier. Simply set a higher local-preference on routes learnt from your primary ISP. A small amount of traffic may still go out via ISP-B due to it announcing a few longer routes to you, but the amount of such traffic is usually trivial.