Oil based Kilz will work great on your trim. Preparing your trim for painting is very simple.
Clean the surfaces if they are greasy or dirty. Usually mild soap and water is all that is needed. Rinse well and allow to dry for a couple of hours. An additional cleaning can be done using a liquid sander/deglosser. I like a brand called Paso; it has a strong solvent smell but works great.
Sanding isn't necessary if the surfaces are in good shape. If your trim has excessive brush marks or chips in the paint then I recommend sanding to smooth out these areas.
All minor repairs need to be done before priming. If major repairs are needed, i.e. Applying wood filler or Bondo to your trim, I recommend priming the surface before the repair and priming the repair itself when finished. This will make sure the patching compound is fully adhered to the surface and sealed for painting.