From what I know it will depend on what sort of paint is already on it, sheen, how dirty it is, etc. will determine how much prep work will be needed.
I'm no where near being a pro but this is what I generally do. I will wipe down the surface with mineral spirits and then use a fine sanding sponge and give the molding a light sanding to knock down any irregularities. Plus of these type of sanding sponges is that they conform to the molding contour. Fill in gouges, holes, etc. with wood putty, unless going for that rustic look, let dry and then re-sand these areas. Clean, prime and then paint. Be sure to use high quality brushes and paint. Makes a huge difference in the final results. I used to go the cheap route and then I tried with higher quality brushes and paint and noticed a significant difference in the final results.
I've often just ignored crevices I couldn't get into with either the sponge sanders or regular sand paper. May be a tool specific for it or something a pro can advise on.