I'm no nail artiste, but I do enjoy dabbling in some amateur nail decor from time to time. My favorite tool to create clean lines is painter's tape. You can pick up a roll for under five dollars and it will last an entire lifetime of nail painting! Below I used Maybelline Color Show Nail Lacquer in Bold Gold as a base with American Apparel nail polish in Passport Blue on the tips!
I tried to go for a more abstract look for this one so I painted my nails a natural sheer pale pink color (OPI in Bubble Bath) with Maybelline Color Show in Bold Gold for the design. Once the base coat has completely dried, I tape off the areas of my nail that I DO NOT want painted with the accent color and paint away! Try to paint the accent color one nail at a time, removing the tape once you have reached the opacity you want per nail. I wouldn't recommend painting your accent color on all of your nails and then removing the tape once your done painting as you run the risk of letting the accent color dry on the tape too long which will cause it to rip and your lines may not be as clean. The goal is to remove the tape slowly before it has a chance to dry. So paint and remove tape one nail at a time :)
Be sure to seal your design with a good top coat. I always get compliments on how shiny my nails are when I paint them and I owe it all to this: Seche Vite Dry Fast Top Coat. I love the thick and shiny gel-manicure-esque finish you get, plus it prolongs the wear time on even the cheapest of nail polishes!
Happy nail painting!