First decision... how many lanes? For a home track, two lanes is fairly standard..... but three lanes is even better. If you have enough space, go for the extra lane. It takes more room but the added competition is worth it. Start with a long straightaway and then start adding corners. Try to make every corner a different radius. Why repeat? You will probably want a tight hairpin and a big sweeper somewhere. The fancy corners are the "increasing radius" or "decreasing radius" corners, where you change the radius (and also the centre point) part way through the corner. These corners can be diabolical to drive. Instead of one 90 degree corner, consider two 45 degree corners with a short straight in between, or even better, one 40 degree corner and one 50 degree corner (for the variety). This requires more driving skill. Make sure that you have easy access to the spin off areas so that the corner marshals can reset the cars easily after a spin. If you are really ambitious, you can add a chicane where you narrow the lanes on a straightaway to prevent passing. At the Yarker Memorial Raceway, I narrowed the lanes at the hairpin just enough to make passing difficult (lots of wheel bumping). This requires that you use a different centre point for the arc of each lane. Try to make as many left handers as right handers, this minimizes any lane advantage.

You might consider an overpass, as seen in Valleyview Motorsport Park. This is the best way to even out the left/right corners and the elevation change adds to the realism.
 
 

An overpass is actually not all that hard to build.  It depends mainly on your track material, the more pliable the product the less likely it is to snap when bent.  It's easiest if you building it in conjunction to one of your straights that way you can have a fairly gentle transition on & off the bridge.

Do your entire track as one level to start with (I started by having a level crossing where the bridge is now), once your happy with that, cut out the shape of the upper portion of your over pass.  Allow yourself as much length as you can for entry & exit so the slope is gentle and you're not creating too much stress on your road material.   Also leave enough clearance in the underpass for your tallest car and then add 1/2 an inch.  In the picture above the Demon Tweeks 911 (red car yellow lane) would sometimes lift a wheel on the corner approaching the underpass and then hit the bridge abutment with hits roof.  I bought a "Power and Glory" Tyrrell 007 and it wouldn't even go under.  Lucky they cleared the overpass itself and the abutment is just cosmetic (5 minutes with a jig saw fixed it).   Don't forget about clearance for cars to slide wide as well.  In the blue lane (above), open wheeled cars would sometimes hook a back tire on the bridge abutment (below the Ferrari), a guard rail from an old AFX track extending from about half way to the pointing figure, through to the other side of the bridge fixed that.

Lay the overpass on the track to mark where the slots should line up then do your routing.  Use a circular or table saw to cut away the underside of the overpass at the ends to make a wedge where it meets the track, that way the transition will be smoother.  Place wooden spacers on either side of the underpass to give enough clearance, and maybe a couple of spacers on the on/off ramps, then use wood screws to attached the overpass to the track surface.  Take care not to bend the overpass to the point of breaking.

Once the overpass is secure, apply layer after layer of drywall joint compound to the transition until it's a fairly gentle slope.  After each layer use a screwdriver or stiff cardboard to carve a slot that joins the slot on the track with the slot on the overpass.  The thicker you apply the drywall compound the longer it will take to dry but eventually it will.  Make sure the cars' slot guide doesn't bottom out in the slot when coming on & off the bridge.  Sand 'til smooth (including the slot) and paint.