Hi Daniel,
Using Multiple ISP is reference to BGP Multihoming ,
To manipulate to advetisement in BGP you should user BGP attributes . Below are the detail .
From Inside to Outside use : Local Prep
From Outside to Inside use : Prepend AS
In your senario if you want to user ISP B as a primiary link then you should configure AS prepend with ISP A . So over internet the route will prefered from ISP B becasue of shortest AS Path.
Please refer for sample lab for same : http://showipbgp.com/bgp-configurations/cisco.html
Regards
Chetan Kumar
http://chetanress.blogspot.com