To quote Nancy Reagan, "Just say No" and to quote Phil Knight, "Just Do It". There are a lot of complicated responses on this thread, enough to confuse me. It's simple, just say no to what you don't want to do or does not make you money. Just Do what you want to do and make money doing it. Remember, you got in the sign business because you were captivated by it, you were creative and you thought a little different then the others. You realized the huge cultural impact design and signage has in this world. The joy of creating. Oh!, wait, wait!, did you get in this business because the profit margin looked good. Because, all I have to do is make a drawing and the MACHINE does the rest. Prints it right out.
A few years ago, I raised my prices, qualified each customer and their potential for profit or trouble. I quit handing out business cards and referred anything I wasn't comfortable with doing to another shop. I'm good at what I do and I charge for it. A single decal for your truck window, try Fast Signs, Express Signs, or another of the shops set up by entrepreneurs who got into this just to make money. See, I love designing, I love fabricating, and if I don't have to dig too deep, I love installing. Taking completion photos is as rewarding as depositing the check. If getting rich was the goal, I would have chosen any number of other occupations.
Go into the shop in the morning and enjoy your profession.