Be Fair
If you are building a slot track to race with friends, you should try to keep each lane fair. For example, you shouldn't give yourself an unfair speed advantage by creating a lane with fewer curves and more straight-aways. You should also avoid making one lane more technical than the other. A lane with six loops will take longer to complete. To keep things fair, time each lane's top speed. If one lane is constantly faster than the other, take a second to figure out the reason. Fix any speed discrepancies in your lanes to make your slot car track more fun for everyone.
Cross Overs
One of the most thrilling moments in a slot car race is a cross over. A cross over is when two lanes cross each other. This can lead to many close calls as your cars narrowly miss each other, and can even lead to crashes. Crashes are fun, but they tend to slow down the racing action. Try to minimize cross overs. Though they are fun in small numbers, too many will lead to too many crashes which will quickly become irritating.
Variations
Perhaps the most important tip in slot track racing is to design variations in your tracks. A single oval track makes for fast paced races, but this quickly becomes boring. Vary each of your lanes with technical sections. Hairpin turns, loop-de-loops and chicanes make excellent variations for your track. However, you should also include straight-away sections to help build speed. If your lanes get too technical, your cars won't build up enough speed to get through the track. Start out with a straight-away section leading into some large sweeping curves with technical twists and turns in the middle sections of your track. This leads to a fast-paced and varied track.