PLA works best if it cools quickly so you need to print on a cold build plate and it will not stick to Kapton tape like ABS does. The best surface for printing is blue paints tape that you can buy from hardware stores. Also because the filament likes to cool quickly make sure you leave the lid off and the door open while printing.
Another tip which can help when printing PLA in the MakerBot Replicator 2X is to rub a a little canola oil on the filament as you are loading it. The oil will just coat the surface of the PLA and help stop the heat rising up into the filament so it doesn't soften as much where the drive gear grips. You can use a printed part like this which you can put a little bit of sponge inside which you can put oil onto and it will keep the filament clear from dust and oiled for printing.