If you wanted to decrease your velocity, you can try combining this method with changing springs, adding a regulator, and/or using a dialable bolt. If you don't have any of these items, you can try cutting off a bit of your main spring. Be warned that this is irreversable. Best to do this on a spare main spring and trim only a small portion of your spring at a time.
You'll probably want to use stiff, durable washers. Try PTFE, black nitrile (buna-N), or polyurethane. If you're desperate, I guess you could even use metal. But keep in mind that the washers need to be easy to install and remove. Follow the same procedure as you would to change the main spring.
The person who first told me of this technique in 1996 recommended that I run my Sterling on liquid CO^2 (siphon tank), and write down the resulting shooting velocity on each washer. (Of course you'd increase the number of washers in your gun to increase velocity.) I was told that running on liquid improves consistency without hurting efficiency too badly since you're using a pump gun.
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