Seriously listen to Jonesey. If you need an instant mix for a specific headphone setup and don't want to screw around with your levels insert sends on everything (that's needed for tracking) and send them to a bus.
Solo the bus and create your headphone mix using the Sends levels on each track.
If you already are using busses for each set of instruments (Drum Bus, Guitar Bus, Bass Bus, etc) then it's even easier because you can just insert sends on the busses to send to your headphone mix bus (instead of on every track).
Once you are done tracking you just mute (and unsolo) the bus. That way it is not blending in to your general mix. You can set up multiple headphone mixes this way by using multiple sends for each artist without screwing up your general mix or screwing up previous headphone mixes.
Another way is to do a "Save As", mix the song as is in a way that works best for the artist you are recording, export it then import it into a new track in your original project. Solo that track and whatever track(s) the artist is recording into then you can turn just that export track up or down as needed. This however means you can't turn things up or down individually (because it's a whole mix) but it's a simpler scheme if your artist doesn't need things constantly being readjusted (which unfortunately they usually will).
Cheers.