I was always told to "sell the job, not the pieces". As often as you can do it, sell your work as a complete job, materials and labor, in a bid or quote. Even to the GC. All GC's mark up their subs cost, mostly to cover overhead and supervision. Try not to let others provide materials, since you can't control the quality of the material, and you can't make any profit from providing it, either.
I was also always told that if you look at five jobs a week, and get more than three of them, you're too cheap. Figure out what you have to have to stay afloat, then add some for profit.
