Let me start off by saying that if your horn is in need of a cleaning you should take it to a reputable repair shop and get it professionally cleaned...but that is not always an option.
Even at home I suggest a 3 part cleaning.
Soap and water - de-greaser (leaves film)
Distilled Vinegar - Acid (for lime and scale)
Solvent - Denatured Alcohol or NAPTHA
Firstly, you will need to disassemble the valve completely.
Take a paper towel and label 1,2,3 and so on for as many rotors or pistons as you have. All of the parts to the piston/rotor should go back to the corresponding number. Wear patterns to the parts are important for feel and playability.
A scrub brush will help knock away any build up. make sure you watch out for what the material is made of. I would suggest fine to medium bristles that will not scratch or mar the metals.
The stem - vastly made of aluminum reacts to acids so this should be cleaned in soap and water.
(make sure you take off stem felt.)
Valve guides - Cleaned in soap and water.
Springs - Vinegar
Pistons/Rotors - Vinegar
Bottom Caps - Vinegar
Start off with soap and water - This will de-grease the parts that have oil build up.
Only use vinegar on parts that can handle acids.
-Leave parts acceptable in acids for 5 minute increments. Take out and clean and if needed put back in for another 5 minutes.
Clean the inner casing with solvents as many times as needed to make sure there is no residue.
Oil and reassemble. Get back to playing!