Well, I have a TRICK! Following is a tutorial to show you how you do it.
Step one.... Choose your two colors. Paint the whole surface with the lighter of the color. In my case, white. Let it dry very well.
Step two... use a piece of the painter's tape as a spacer and make your lines. Be sure to press the edges of the tape down very well.
This is the trick!
Next.... paint over the edges of the tape with the same light color. Weird, right? But this seals the edges of the tape and allows anything that is going to seep under to match. Cool, right?
See, I'm painting over the edges with white.
Here are the edges painted white. Now let it dry, again, very well.
Now paint your other color over the white and over the tape edges and let that dry.
Remove your tape.
And there you have it. Nice crisp stripes.