NOTE: I did not create this mod, so I can't offer much support. I can only tell you what worked for me.
This changes the disguise system so that it is closer to the one in Hitman 2, Contracts, and Blood Money. You can find it here. Download it, unzip it anywhere onto your hard drive, then open up the the 'script.var' file in a text editor and fool around with the values. I found the following to be pretty balanced:
You probably want to keep the second value somewhere between 0.25 and 0.50. This changes the detection system so that you will only be detected in a disguise if you stand pretty close (3-5 ft. or so) to someone in identical clothes, as opposed to how it is by default where you can be spotted from 50 feet away and further. I have only tested this on Purist, so I don't know how it affects detection in the other modes.
Once you have the values set to your liking, save the file, then run the executable called 'Configurable_Injector'. It should pop up a small window. After you see that, start Absolution. Once the launcher appears, the Injector window should disappear. Go ahead and start the game and load up a level. After you are in game, press the F9 key, and you should hear a very faint ding/beep. I'm not 100% sure, but I think you might need to hit F9 again after reloading a level or starting a new one. You will need to run the Injector program every time before starting Absolution (as long as you want to keep using it).
If you open the README file you can see a list of the hotkeys for the mod, I haven't used any of them (except for F9) so I can't speak to how they work. I do NOT recommend cheating for the sake of getting on the leaderboards, the only reason I'm reposting this is because it has made the game significantly more enjoyable for me, and hopefully will for you too.
EDIT (12/5/12): Link updated to newer version of trainer