I currently carry two walkman type cd players as back up. Quick to set up and fit behind the front seats of the van so very unobtrusive. Not practical for daily use, but when it comes to playing music a CD Player is a CD Player, and the only person who would know is you

. Currently changing over to Laptop, so will use the laptop for the main audio and then keep my present dual cd as "back up" along with a case of CD's and resign the portables.
Also keep a back up 2 channel mixer in the same place, although my faithfull old Cloud DM1200 has been bombproof and never let me down.
At larger gigs I use two amps to Bi-Amp my Bass & Full range so if one amp failed i'd just hook up both Bass & Top speakers to the one remaining amp, and run in passive full range mode. Even if I'm only using the full range speakers at a small gig, I still carry both amps in the same flightcase, which would serve as back up.
If you don't have any back up then you seriously should re-consider this. If a mixer or amp failed and you had no back up, then technically the evening is over

refunds would have to be issued, and your name would be mud - especially at a Wedding where the D.J can literally make or break an otherwise perfect day.
Any D.J who goes out on a regular basis and doesn't have any form of backup, or contact with another D.J locally who could help, is asking for trouble. Just because it hasn't happened yet, doesn't mean that it couldn't happen tomorrow

and you could ruin an otherwise untarnished reputation if the client you are working for, when it all goes wrong is the local town gossip!