There are many different techniques to treat your build platform to help your prints stick better. Here is a list of some of the most common ones; you may have to experiment to find the right technique or combination to fit your printer best.
Using Blue Painter's Tape
Blue painter's tape is most commonly used with the unheated acrylic platform. Especially if you are using PLA, the filament can stick to the acrylic build platform too well and be hard to take off. Blue painter's tape offers an easily removable and replaceable alternative to which filament still adheres well. Cover the bed with strips of tape laid parallel to each other, trying to avoid too much overlap or too many gaps. This can cause a slightly uneven print surface because of the seams, but for many people this is a minor issue.
Cleaning Your Kapton Tape with Acetone
Filament will adher better to clean Kapton tape than one that has been covered with dust, fingerprint oils, and other debris. Take a clean rag or towel and soak it in acetone. Make sure your heated build platform is cool to the touch. Next, vigorously wipe your Kapton square until the surface has been completely wiped and clean. You should wipe for approximately 30 seconds.
Buffing Your Kapton Square with Sandpaper
By gently buffing the Kapton on the heated build platform, you may create an extra "grip" for your print. To buff the Kapton, it is a good idea to use sand paper with a grit rating of '60'. Buff in a circular motion across the entire platform. Alternatively, you can buff in two perpendicular directions to create a "cross-hatch" pattern on the Kapton tape. Once you have finished sanding, you should wipe the build platform with a damp cloth to clean all the sanded dust off. Repeat as needed, though be careful not to scratch a hole through the Kapton tape.
Applying Hairspray
Hairspray creates a texture on the build platform which may make filament stick better. In general, you want the ones with the strongest rated hold possible. One of the more popular brands is Aquanet, which can be found in most drug stores.
To apply the hair spray, move the extruder out of the way to the home position and lower the build platform. You want to avoid application during printing, as you could clog the extruder with the hair spray. Spray a light, even coating onto the platform.
Using an ABS Slurry or Glue
As a final resort, you can apply an ABS slurry to your build platform as a type of glue. You can make your own ABS glue out of acetone and scrap pieces of ABS filament by following the directions here. This is a final resort, however, as it can be extremely difficult to clean your bed and remove prints after application.