Hi,
Valves and cylinder cleaning can be done yourself. We have been doing ours now for a few years.
Buy the Oxygen hackers guide from this link and away you go.
Airspeed Press Homepage - Books For Serious Divers
All O rings for your valves can be bought cheaply from an industrial bearings supplier.
Oxygen compatibale grease can be bought from these guys at a reasonable price.
Welcome to SEA-LONG Medical Systems, Inc. - Parts and Accessories
Use Simple Green to degrease your cylinders which can be bought from auto shops.
You'll need a tooth brush and an old hair drier with a home made hose attachment to go down into your cylinders to dry them out straight away to prevent flash rust occuring. Alternatively a cylinder of air will do as well.
When rinsing your cylinders use plenty of hot water for final rinses and run water continuously into your cylinder until no more suds come out.
I suggest degreasing one cylinder at a time from start to finish to avoid flash rusting.
Valves are even easier to pull apart degease, change Orings, lube and put back together
Have fun.