What type of paint are you using? Acryl, enamel... what brand? Are you priming?
What type of masking tape? I have found that the cheaper masking tape tends to have a little bit stickier sticky side. How long are you leaving it on b4 removing?
All of these are contributing factors... The Tamiya masking tape, while a tad more expensive than the plain stuff, is awesome IMHO.
prime your model, paint, let the paint cure untill you cant smell it anymore, clear coat, let clear coat cure, mask, paint, pull masks off as soon as possible.
i prime with rustoleum auto primer, paint with tamiya acrylics, and clear coat with testors lacquer clears, mask with tamiya masking tape,