When you are orbiting Kerbin, you can just set the Mun as target to help getting into an equatorial orbit. If your are in orbit around another body, say Minmus, you can open up the debug menu (Alt+F12) and look at the line in the top right that says "Lat." If it is decreasing, point normal (if it is increasing, point antinormal) until the number is nearly at 0. When it is nearly zero, increase the throttle and immediately cut it. Keep repeating this until your latitude stops straying too far from zero.