[shamefully ripped from mailing list, I'm keeping it here for my personal reference]
Put a "tail" at the beginning of your print. I never print without a tail, even if I have a raft. Basically at the beginning of your gcode around the point where your print starts change your gcode from this
What causes lack of sticking t is two fold. 1st the plastic in the tip of your nozzle is not 230-240, it's more like 200-210, it's really not sticky. The extruded plastic isn't up to temp till you have extruded 1-2cm. The tail gets the extruded plastic solidly up to temp before you print your interface layer.
Second when printing raft less your Z needs to be very squished into the plate, the tail gives you 10-20 seconds to jog you z axis to the perfect level. And it lets you find any imperfections in your leveling before you loose the print.