The easy answer is a good transparent limiter, or as close to that as you can afford. Even the best ones will color your sound when you are hitting them really hard by adding some harmonic distortion. This can be a good thing or something you want to avoid.
It never hurts to have a singer with good mic technique as well. If the singer lacks this it doesn't hurt to explain that it might be a good idea to back off the mic a little at times. Do this carefully though, singers can be really sensitive.
Also keep in mind that they have to be hearing enough of themselve in the headphones for this to work.