Make your kitbash look like one model: limit the palette
The quickest way to make a kitbash look like it was always meant to be one model is often the paint, not the parts. In this one, Trovarion shows how working from a limited palette (picking a handful of colours from the colour wheel with a "gamut map" and mixing everything else from them) makes a miniature look far more harmonious. He paints a whole figure from a single themed set and keeps a single saturated blue as the star. It's a brilliant mindset for conversions: when the arms, heads and bits all come from different kits, or you've mixed plastic parts with the 32mm minis we print here at Nebulous Models, a tight shared palette ties the lot together.
Do you plan a colour scheme before you start a kitbash, or work it out once it's built? How many colours do you usually allow yourself?