I do not know very much of rally, but for what I see in my club, it good to dismount and clean all the cars after the event.
As for the setup, I know that for lubricating the axles, pinion and crown it must be the most reduced possible. For the motor, in Ninco type motors if they are standard ther is no problem. As for the SCX motors the holes can be closed with a tape. an other thing important is the braids, which must be opened in 2 to 3 mm in the end.
About the track, after you put the flavour, you must clean the contacts in the track with a brush.
There are also difrent type of tyres for tracks too dirty, less dirty and so on. You can see in this site of a Spanish manufacturer of rally slot goods
http://www.pekaese.com/
diferent type of tyres, chassis and other things specially done to rally slot.
Cheers,