You can find products on the market that make it much easier to create stripes and other decorations with a contrasting color. Generally shaped like your nails, these little design aids have a sticky side that you attach once your first polish has dried.
Another option is to use tape to create distinct lines. However, this really limits your design options. In a pinch, regular tape can be helpful for adding a bit more character to your nails.
The popularity of stripes has caused manufacturers to redesign nail polish brushes. In addition to the traditional brushes, you can find an array of brushes designed for specific purposes, including the creation of stripes and dots.
Nail tape can be used effectively to create fantastic designs as well. Available in practically every color imaginable, including holographic designs, these are excellent for creating nail designs.