Hello,
And why this is an issue? This is how internet works, BTW.
Anyway, Your choices are:
1/ announce less specific prefixes to Your backup ISP (say, 3 * /23) and more specific (say, 6 * /24) to You main ISP.
2/ prepend Your prefixes towards backup ISP, do not prepend towards main ISP
3/ use community to influence Local Pref and/or prepend in Your ISPs - example here https://www.us.ntt.net/support/policy/routing.cfm#communities
[1] will give You the best results but only if Your CIDR blocks are /23 or greater. Most ISP do not accept inbound prefixes with /25 and longer masks.
[2] works only to certain extent depending on where Your main sources of incoming traffic are.
[3] works if Your main sources of incoming traffic are within Your ISP and 1-2 ASN hops beyond
I am sure other people will chime in with advice to use MED - MED is not transitive and does not work beyond Your 1st ISP ASN hop.
HTH
Thx
Alex