One of my good buddies, Christophe, is renovating his garage into a modeling studio and he makes his builds come alive by adding electronic and electrical hardware, which fits really well into 1/24th & 1/25th scale builds. 

I look around at all kinds of things that get tossed everyday as junk and rip them apart to see if I can't find small pieces that will really make a build unique.

From a painting stand point, I think polishing is a too often skipped step that can really make a difference in the way your car will display.  My favorite rubbing compound is Tamiya, they come in three different grades, Coarse, fine & finish.  You can find them pretty cheap here on eBay and since I'm in the Midwest, I like to buy from the states because it's cheaper to ship and gets here pretty quick with Priority Shipping (2-3 days)

What building tips do you have to make your builds come alive?