Otto,you can buy a very simple attachment that uses compressed air to hold the valve shut,just scres in the spark plug hole and pump up the air,hold the valve shut while you dissasemble it.not much money and works well,make sure you have the piston at TDC before you do this.
The air thing in the spark plug hole will keep the valve from falling into the cylinder when the valve is removed, (a length of nylon rope fed into the spark plug hole works too) but you will still need something to remove the spring.
Here's a good tool for that... It screws onto the stud, and you push the lever down to compress the spring. Once compressed, it locks lock in the down position. I have the exact same tool (only mine is made by Manley) and it works great! I use it all the time. A word of advice. Hit the retainers with a hammer to break them free from the valve locks before breaking them down. The handles rotate around in any direction, and the threaded part that screws down onto the stud flips over to do both 3/8" and 7/16" studs.
It's available from Summit Racing at the below link for $49.95...
I have used a piece of bar stock with a slot cut in it and bent at an angle. Put a nut on the end of the stud, slide the "tool" underneath the nut, lever them springs right down. This was with stock springs and the thing was barely adequate because it would give just a little bit but if it was braced it would have been much better. If I had to do it again I would get the proper tool though.