Our trick is that we are one band with three names. The 1st band is a serious Blues band that only plays festivals and ticketed events. The 2nd band plays blues and rock 'n' roll, and the 3rd band has the guitarist as the main singer and started out playing ska and reggae stuff but has evolved into a punky kind of sound. All the bands have different names.
Also, when our local main venue (which has 3/4 bands on per night) calls to say they've been let down by a band and asks if we can help at short notice, we always say yes.
So now, if a band pulls out, we're the first guys they call. We get 3 or 4 cancellations per month at that venue, as well as the normal pre-booked gigs they give us.