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.