You can also prepend the /24 prefixes you want less per ISP.
This is the more commonly-applied solution, but it's not guaranteed to work in all cases. If the peer applies a local_pref to your announcements which is superior to any other path it may hear your prefixes on - well, then the ISP itself is going to prefer you no matter how many times you prepend the AS. Most transit providers have a policy which starts with this basic philosophy:
prefer customers, then peers, then transits (of course, tier 1 carriers have no transits)
Most of them are happy to modify this policy for their customers who wish it, and usually through the use of communities, but I'm sure that it's also possible to officially request the policy change via customer service / change orders / etc - and have it set administratively (not dynamically with communities) and that's fine too.
I recommend using a combo of communities/as-prepend as the first choice, but if the customer is not skilled with this and can't get the circuit to ONLY act as a backup due to something like I described above, then simply announcing longer prefixes to the preferred carrier is an easy, guaranteed way to get the job done. (but very much a "brute force" technique that doesn't allow for fine-tuning the behavior)