That's really tough to answer not knowing your current skill level or what they are turning out like. Here's a few general thoughts though.
1. for autos, paint is one of the most important things! You really need to have a smooth even paint job. Use a spray can or airbrush and coat with a gloss clear-coat for a nice clean finish.
2. fit of the parts is very important. The parts need to fit together tightly (especially bumpers, body panels, etc) or else they will not look right. Make sure the fit of the parts is solid.
3. no excess glue. This is very important. You don't want glue oozing out from the seams on an auto. Only use enough glue to securely hold the parts in place. Too much glue is very visible and distracting.
4. stance. Often times, the way that the car goes together with the chassis and wheels is not correct. The model will sit to high or just look odd when complete. The only way to deal with this is in the first stages of assembly. You need to do a 'build-up' of the pieces to see how they are going to go together and what they will look like. Then, you modify the pieces or scratchbuild various parts to adjust the fit of the pieces so the 'stance' is the way you want it to be. ie. if the model is sitting too high, you might be able to cut the 'springs/shocks' so that it sits lower.
5. interior - use various 'shades' of your interior color for the interior components. This will create slight variation rather than just a completely monotone look.
I'm just learning myself, but those are the things I've picked up so far...
M.