The secret to nice crisp lines in a paint scheme is the correct masking tape. Standard tan or blue "household" masking tape like 3M from the big box stores will allow the paint to creep under the edge and leave a "Ratty" parting line. Do your initial masking with 3M Fine Line tape in either green or blue. I prefer the blue because it's more flexable if you have curves in your color scheme. The tape isn't cheap and is usually stocked at any auto paint store. Then do the "rough" masking with the household paper masking tape. Overlap the rough mask onto the Fine line for complete masking coverage. Another lesson I've learned is do not use newspaper for masking. It leaves ink on the surface. I use the brown masking paper you find in various width rolls at the big box stores or any good paint supply store.