Brush painting

First, paint the top part of the model one color.  After waiting for that coat to cure (usually the next day), paint the bottom part of the model a different color.  If the color demarcation is supposed to be hard edged, mask the top color with Tamiya Yellow masking tape.  To alleviate paint bleeding, try not to really pile up the paint on the edge of the masking tape.  Brushing away from the tape edge helps.  If the color demarcation is supposed to be soft, use a bit of foam rubber, and carefully stipple the edge with it.  If you keep the foam fairly dry and do overload it with paint, you can achieve a decent soft edge. 

Man I wish they could fix the spelling on that sign!

edit Thanks to MusicCity I have a CORRECTED Ditto Sign! Thanks Scott! /edit 

I'll add that you should paint the lighter color first, regardless of top or bottom, otherwise Abarne was spot on.