Surface Prep is 90% of the job. If tape fails, it's almost always because the surface wasn't clean.
Clean: Use rubbing alcohol (isopropyl) . Soap and water leave a residue. Alcohol removes all wax and grease.
Dry: Make sure it's completely dry.
Warm it up: If it's below 60°F (15°C), use a hair dryer to warm the surface AND the tape before sticking. This activates the adhesive.
Pressure: Press firmly for 60 seconds.
Wait: Don't wash the car for 24-48 hours to let the adhesive cure.