Some things I've tried that have helped:
Lay down a layer of masking tape. Most people who do this use blue painter's tape. The plastic should stick nicely during printing, yet release reasonably easily when you remove the print from the heated bed.
Lay down a later of Kapton tape. The principle is the same as masking tape, but Kapton tape has a smooth surface and is more durable than masking tape. The down side is Kapton tape is far more expensive, and applying it correctly is a LOT more work, since you have to use water and you have to keep bubbles from getting underneath it.
Put some ABS scraps into a bottle of Acetone, and allow the acetone to break down the ABS til you have a slurry. Spread this slurry as evenly as possible across the build plate, and allow the acetone to evaporate away. This leaves a thin film of ABS on the plate, and will release much better than if you print directly onto the build plate. I recommend using clear ABS if you can, since some of it will stick to your print and clear will be the least visible. You'll need to re-apply it regularly, since it will come off with your print where it touches the build plate. WARNING: Use proper ventilation and avoid contact with acetone. That stuff's not good for you. Also it's flammable, so keep a fire extinguisher nearby.
I prefer the ABS/acetone slurry method, but it requires good ventilation and a handy fire extinguisher. Also note that you don't have to print in ABS to use an ABS/acetone slurry; I print primarily in PLA and it makes no difference.
I've also heard of others using a glue stick or some other surface treatments that allow for good adhesion during printing while still allowing for easy removal.