Mike,
I'd venture to say you will get as many different answers to your question as the number of Modellers you ask! I've tried every method and medium there is to thin the stuff down (I use "Pledge with Future Shine) as the available Canadian version these days. My best advice to date, and recently from another model builder is: "Don't thin it", as long as it isn't really old spray it as is. Actually I mostly use a wide soft brush, work quickly, and get really good results, FWIW!
Cheers,
Lance