My two solutions:
All stock parts. Glue in motor, propshaft bearing and rear axel bearings. True the back wheels by spinning them against sand paper until they are perfectly round. Replace tyres and repeat the process trueing the tyres to the wheels. Glue the tyres to the rims around the edges. Make sure that the braids are making good contact by straightening them and flicking them up slightly at the end. Make sure that the body doesn't rattle at all, which may mean having to glue the interior and additional parts (lights, grilles, etc.) to the main body to prevent vibration. Make sure the body doesn't rub on the chassis or wheels at all. If it does, scrape it away with a sharp knife or some sandpaper. Losen the screws a couple of turns, and away you go.
Replace rear axel with Slot.It. Cheapest wheels are standard ones, then lightweight ones, then magnesium. As they get lighter, they get better, but more expensive. Also, replace the front wheels with these wheels. Remove inserts from all wheels. Add Slot.It Zero grip tyre to the front and grind them down to as small as you can get them, round off the edges. Replace rear tyres with aftermarket ones (see below). The gear ratio depends on what track you are running on (see below). Replace motor (see below). Replace interior with a lightweight vacuum formed version. Replace guide with a Slot.It deep guide, and cut it down until it is deep enough to just clear the bottom of the slot. Replace the braid with SCX or Slot.It soft braid. Use silicone wire instead of standard.
Right, now you need to give us some more information. What manufacture of track is it that you are running on? Sport, Classic, Ninco, Carrera or other. What kind of track is it? Short and twisty or long and fast. What's the power like? Are you racing with or without magnets?
If you are racing without magnets, you should probably put around 5 grams of weight around the rear axel to aid traction, though this is not essential.
If you are running on Sport track, use Ortmann, Proslot or Slot.It P3 tyres. If you are running on Classic, Ninco, Carrera or routed use Proslot or Slot.It.
If it's a short and twisty track, try a gear ratio of 9 tooth pinion to 30 tooth contrate, and if it's long and fast try 9 / 24. To start use 9 / 27 for most tracks and work from there.
Motors: Slot.It V12's are good on high power, but not too good on standard power. A Fly Racing motor or NC4 would be better for lower powered tracks.
Hope this helps, awaitng response...