What you can do in the way of stopping a breakage depends on a lot of factors. If there is a big deep clean sided crack behind a flake then you may be able to do something useful. In most other cases a preventative re-enforcement achieve much. If the hold is definitely going to break off anyway then technically by far the most effective thing to do, which will also give the neatest finish, is to carefully remove it, clean it up and stick back on with something like epoxy acrylate resin (stuff used to glue bolts in with). Whether this is appropriate/ethical at any given crag is an entirely different question. If you were to do this then consider hardening the two surfaces to be glued with low viscosity superglue first and before the resin sets throw some nearby sand and mud on it and press/rub in to give a decent visual/tactile match to the rock.
