2. Find a lab you like and stick with it. I worked in a photo lab briefly during college. In a perfect world, we would have spent time examining every image to be printed and balanced the output accordingly. But the truth is, with limited time and lots of jobs, we tended to work more like an assembly line and less like skilled craftsmen. The exception to this was when we were printing images for regular customers. Those photographers who came in frequently enough to be remembered invariably received better service and had more attention paid to their prints. Unfair as it may be, that's often the reality. If you can get to know one of the technicians on a first-name basis, you're set.
3. Know what you want and give clear instructions. If you don't use a lab often, you can get the extra attention paid to your prints that the frequent customers receive by letting the lab know what you expect from them up front. Be specific. The customer who makes clear requests (e.g., boost contrast, saturate the colors, burn this area, etc.) is more likely to get satisfying results.
4. Get test prints. When ordering oversized prints, it's worth every penny of the expense to have test prints made. Depending on the size of your prints, you could be wasting tens or even hundreds of dollars if you get back prints that aren't satisfactory. And as long as the lab followed your instructions, you won't have much leverage to ask for a redo. That's why it's a good idea to have test prints made. Check the test prints for color, exposure and sharpness before the lab makes your final prints. You may spend a few extra dollars, but think of it as a very affordable insurance policy.
5. Ask questions and educate yourself. Don't be shy. Let the lab know what you're after and get recommendations. If you don't know what a particular type of digital output looks or feels like, ask to see a sample. How long will the prints last? What surfaces or substrate materials are available? What's most popular with their customers? One of the chief reasons that people are dissatisfied with their prints isn't a result of the lab not doing its job, but of the customer expecting one thing and getting something else. If you know what to expect from a glossy print on Fujicolor Crystal Archive versus an inkjet print on matte canvas, you'll make better choices and ultimately be happier with the results.