It's tough to say how to get the right sound, because it involves ALOT of patience, and a good ear. If you don't have either, then ask someone else to do it for you.
In my previous posts, you've seen I've mentioned Izotope Ozone and Cakewalk Sonitus:FX as two multiband compressors. Both are great in their own sense (Ozone has a bunch of tools integrated into one interface, while Sonitus:FX has separate plugins for different tools - not integrated).
Izotope Ozone 4 - you can see 6 modules in one, but there are more features than meets the eye.
Both are professional, and not for free (though personally, I prefer Ozone for it's ease of use and sound quality, but Sonitus:FX because they have a 5-band compressor... Ozone has 4 bands)
The Cakewalk Sonitus:FX suite (note - you can use one plugin at a time instead of having to use all modules)
Anyway, I've literally wasted ALOT of time trying to get the best sound for my compressor, but in vain each time. One of the local stations sounds very nice and sweet (heard Absolute Radio and BBC's Radio 1? Some TV channels sound as sweet and sharp too) and I've been trying to get a preset like that, but for a very long time, I guess I was either looking in the wrong direction, or did something miserably wrong!
I searched the net continuously, and eventually came across posts saying the big commercial stations used a product from Optimod. Now I can't pay to get something like that, so I came across another multiband compressor called Sound Solution (make sure you get version 1.3, and NOT 2).
The site has TONS of user-contributed presets, and a quick start: put the DLL in Winamp's Plugins folder, and the presets in Winamp's main folder.
I spent a whole day yesterday figuring out which preset to use, and I came across one. It doesn't sound exactly like an Optimod (and I don't know how they sound) but it comes pretty close to a commercial radio station... if you want more treble, you can tweak it further, but this has an appropriate amount of gain, sweetness, bass, ideal amplification (the volume doesn't fluctuate too much, and something that's quiet should stay quiet - you'll probably like this preset if you're very constantly-changing-volume-conscious - something many stations continuously do and sounds awful in terms of consistency).
Preset uploaded - http://shaheer.a.khan.googlepages.com/ss3.dat should be downloaded, and put in the Program Files\Winamp folder)