Tried so many techniques over the years, but nothing has gained such a good result as with Finess pinstriping tape. It's a template tape with pre cut stripes, you can choose width, how many stripes you like in their catalog or special order to your specific need. This is one of the bikes i did with it:
Hope you enjoy some pics om my restored 1937 Crescent that was finished last friday. I have done everything by my self except some chroming. The striping on the rims and fenders are done with Finesse pin striping tape, ordered to correct width and space of the lines. I have been looking for...
thecabe.com
Only downside is that it's time consuming to apply straight and neat, but well worth the effort.