Basics:
Keep to the instructions as much as possible, but watch out because some instructions will accidentally omit steps or place them in the wrong order. Definitely dry-fit everything before gluing. And if you're installing lighting - wiring, LEDs, chips - check that everything will fit inside the model before assembly.
Fill and finish seams - I use gap-filling Kwik-Gel 4 for seams/assembly - but you will have to shoot primer to make certain you're getting all of it right; keep it to a light coat, that should also reveal any scratches, pits, etc.
I have airbrushes (and can do a pretty good job with them) but prefer to use spraypaint; less work and more consistent.
Warm spray cans in water and shake well before using. Spray light coats of primer and let air-dry for at least an hour; Krylon primer takes longer so be careful with that one. Light coats also for finish-coats, easier to correct than cleaning up drips and runs that get away from you. Can always do a heavier final coat to even out everything once you're sure primer isn't showing through. Blue painters tape or Tamiya tape for masking.
(Personal note: Duplicolor spraypaint for cars is really great to work with because they dry very fast and take multiple layers of masking without lifting or tearing at edges if you score the tape correctly.)
Clear gloss coat the model/assemblies before applying decals, decals will not adhere well or at all to flat/dullcoat finishes. Once you've applied the decals, apply another coat of clear gloss, then you can go for final coats of flats for weathering, etc.
Generally speaking, apply a couple light coats of clear gloss coat (let dry at least 24 hours) to the decal-sheets before cutting up and applying; this goes double for any decal sheets in the Polar Lights and Round 2 Trek kits, which have totally come apart on me used straight from the box.
For detailing, I use a lot of techniques, and in combination: acrylics, dry pastels, pens, Pentel brushpens (get the 36-pen set - has everything from fleshtones to different shades of grey), charcoal pencils, mechanical pencils, colored pencils.