Adhesion problems (prints not sticking) are very frustrating, but one you figure out how to make your prints stick... it's awesome. This checklist can help you ensure that your prints will stick.
1. Dial in your printer for a perfect first-layer height.
Good: A clean bead of plastic with a bit of "squish"
Good: Infill beads overlapping and contacting each other.
Bad: Beads that are smashed into the build plate.
While this won't cause adhesion issues, it will cause the first several layers to be wider than they should, and the print will be difficult to remove from the build plate. It can also damage your printer. The nozzle is too close to the build plate.
Bad: Beads that are thin and don't stick to the build plate.
If the first layer aheres at all, the print will probably curl or pop off the build plate mid-print. The nozzle is too far away from the build plate.
2. Use blue 3M #2090 painters tape, or Print Bed Tape Squares available at printrbot.com. These tapes have been tested for good results. Other tapes (even just other brands) may not work as well, but feel free to experiment.
3. Use a higher temperature for your first layer, and then back off to cooler temperatures for the rest of the print. 195-205 C for layer 1, and about 185 C for all other layers.
4. Wipe the tape with Isopropyl Alcohol before you start the print. Instead of prints peeling up from the tape, the tape might end up peeling up from the bed!
5. Leave the fan off for the first 15-20 layers. You don't want the print to cool too rapidly. If there are small details, overhangs, or bridges within the first 15-20 layers, you may have to make a compromise.
6. Use the hairdryer trick. Get the bed pretty warm (uncomfortable to hold your fingers on) with a hairdryer while your hotend heats up to start the print. Every 2 or 3 layers, warm up the print with the hairdryer until your fan turns on after layer 15-20.