SP,
Are the heads on the engine or have they been removed? if they are on the engine you will need the spring compressor, the air fitting that goes in the spark plug hole. It uses compressed air to hold the valve in the head. If the heads are off then a simple spring compressor that holds the valve face on one side and comresses the spring on the other side is the ticket. I would also recommend a small magnet to retrieve the two locks as they will fall into the most inaccessible areas.....ask me how I know. Bottom line is Todd is right you will need the compressor tool. If you don't have it and you simply want to change the springs AND the heads are off then any good machine shop will usually only charge you a small fee to change them out. If the heads have alot of miles on them I would also recommend checking the valve guides for wear. You would be really mad to put it all back together and watch it smoke becuase the worn guides were leaking oil into the combustion chamber.
Eric