As suggested by the other commenter, o-rings are the best cheap solution.
Another option that works if your keyboard has a hollow body is sound-dampening insulation inside the frame.
I have a Kinesis Advantage that has a ton of negative space inside and simply putting some non-conductive foam inside the shell made a huge difference in the amount of noise it makes
I feel her pain. It took me a long time to get comfortable with the Kinesis, and even longer to be able to go back and forth with my gaming keyboard seamlessly