Air Asia is the only one to consider if you are looking for the really cheapest flights Bangkok to Phnom Penh. The biggest problem with these cheap flight are that you have to set the alarm clock for hideously early from Phnom Penh or Bangkok. Another option to consider that will cost more but you get perks as well is Bangkok Air they give you more luggage allowance and they don't fly so early in the morning. So if you want a discount flight to Bangkok to or from Phnom PenhCambodia, that's the best deals on airfares.
Another place to look is the Travel Agents they sometimes buy blocks of tickets for specific cities when they are on special. Discounts airfares can be very good if the flight tickets are about to expire,
Booking early on these flights is a definite advantage. the very cheap flights are snapped up pretty quick. If you know that you need to fly between Bangkok and Phnom Penh watch the Air Asia website and the Bangkok Air website for discount airfare specials.
A few new software programs that I like have come out that were developed for real people like us who like to budget travel but are not business travelers on the company dime travel agents defiantly don't like these programs. Check out Sidestep,Kayak, Mobissimo and FareChase for the cheapest international airfares ..The new ones that you can use to check the flight fare info against are FareCast and FareCompare which use crystal balls and such to get you a fair airfare. I was looking to go to Manila and they wanted 500 to 700 for the flight I found a work around that only cost around $100 and I can see Kuala Lumpur as well . Was looking for Tokyo to Beijing was being quoted around $250 will end up paying with China Southern Air