Ez3D

This trick is very simple.  Simply apply an initial coat of glue stick onto your build platform, vigorously.  Purple Elmer's glue stick has been known to work best.  As you print, the glue will likely come off with each print, simply add another layer each time to maintain the sticky platform.

Glue stick works well for both ABS and PLA.

One of the more advanced, but best, methods of bed adhesion is ABS slurry.  If you are an ABS user, over time, you can use your failed or unwanted prints, or scrap filament, to create slurry.

Fill a mason jar with a small amount of acetone, drop in your ABS filament or scrap parts, and close the jar.  Over the course of a few hours, the parts will melt into a thick paste.  If the objects don't completely dissolve after a couple of hours, you may need to add more acetone.

To use ABS slurry, first heat your build platform up to 100º Celsius.  Once it's fully heated, you can then use a paintbrush to apply the slurry onto the build platform.

ABS slurry works well for both ABS and PLA.

Blue painter's tape is also a well known adhesive for 3D printing.  Simply cover your build platform with the tape, and your PLA prints should have a much better time at sticking.

There are differing reports on whether or not using the heated build platform is a good idea with the painter's tape.  Some have reported that using the heat makes it difficult to then later remove the tape.  We have found that using a small amount of heat, like 50º or 60º, works best for PLA.  You will likely want to try both, and see which works best for you.

Painters tape does not work with ABS, this tip only works for PLA.