I've been doing a lot of maintenance on my '87 Samurai and one of the goals was to change the valve stem seals.  But after getting the valve cover off I realized I would have to take off the rocker arms.  But after some inspection and some dumb luck I discovered you can change the valve stem seals with the rocker arms still in place.  My old seals were stiff compared to the newer, more supple seals.
I filled the chamber with rope by feeding it into the spark plug hole while the piston was down.  Then I rotated the crankshaft so the piston pushed the rope up into the valve I was working on.  This way it holds the valve in place so you can compress the spring and remove the keepers.  I learned it from a wise mechanic friend.  You can't do this with compressed air connected to a fitting in the spark plug hole because you need to be able to let the valve go down a bit in order to remove the spring and install the valve stem seal.
WARNING: Using this method, if you're not careful, I think you can shove the valve all the way into the cylinder. The only way to get it out would be to take the head off.  I did it successfully on all 8 valves.  They are nice and big and simple. Not like my 16 valve honda...I used the same method on the honda successfully, but I took off the rocker arms.   

The only novel tools you need are:
- a valve spring compressor (check the PICTURE, OEM brand # 27040, I loaned mine from Autozone)
- a piece of clean rope small enough to fit in the spark plug hole (I had a piece of bungee cord) check the PICTURE to see the rope in the spark plug hole
- small diameter stick to fit through rocker arm adjuster hole ( I used half of a chop stick )
- small magnet on the end of a stick (you can find these at car part stores)

1. Remove valve cover
2. remove spark plugs
3. remove adjusting screws from rocker arms
4. rotate the crankshaft so the pistons for the #2 and #3 cylinders (the two middle cylinders) are at the top of their stroke (look through the spark plug hole with a light and you can see them reach the top).  This means that the #1 piston is all the way down
5. look at the cam lobes for the #1 cylinder (closest to radiator).  They should not be in a position to be lifting the rocker arm. Check out the PICTURE and you can see how the lobes should be positioned.  If they push on the rocker arm, they will push the other end of the rocker arm down so that you will not have enough space to remove the spring.
6. feed as much bungee coard into the #1 cylinder as you can. 
7. rotate the crankshaft clockwise until it stops. The rope is now touching the bottom of the valves and will hold them in place.
8. use the spring compressor to compress the valve and remove the keepers with the magnet. Check the PICTURE below.

Continued from above:

9. now you need to remove the spring.  Use the stick and put in through the valve adjusting hole in the end of the valve rocker arm (check the PICTURE). SLOWLY move the crankshaft counter clockwise and at the same time push on the stick to force the valve into the cylinder. This way you move the valve down to make enough room to remove the spring.  BE CAREFUL because you can go too far and shove the valve into the head and the only way to get it out is to take the head off... Danger! Danger Will Robinson! (flailing arms)
10. remove the spring and spring retainer
11. remove the old valve stem seal by prying the bottom with a big flat-bladed screwdriver followed by pulling on it with pliers. Be careful not to scratch the valve stem.
12. put oil on new vavle stem seal and gently slide it over the valve and push it down until it stops.  Now use a socket ( I used a 1/4 inch drive Craftsman 12mm socket) to push the stem seal into place - check the PICTURE.  It doesn't take much force, you will feel some clicks as it pops into place.  Just to make sure its on I will try to pull it with my fingers after I'm done pushing.  Sometimes the seal can go on crooked. That means that one side is not pushed on all the way.  I push on all sides to make sure this doesnt happen.
12. put the spring and spring retainer back on
13. the TRICKY PART!!! Rotate the crankshaft clockwise again. You need to force the valve back up all the way so you can install the valve keepers.  What happened to me on most of the valves is the valve would not go all the way up! I think that when I made the valve go down, some of the rope got between it and the valve seat and prevented it from being pushed up all the way.  So to fix this I had to lower the cylinder by rotating the crankshaft counter clockwise,  take out the rope, and then pull the valve up with some needle nose pliers. I had to pull the valve up with the pliers by reaching through the spring coils to grasp the valve stem.  And then re-insert the rope and rotate the crank clockwise to force the rope up on the bottom of the valve to keep it in place as you compress the spring and install the valve keepers.
Installing the keepers is nasty business. I have a bad back so I had to take many rest breaks because I had to bend over the fenders.  It takes patience and good light. My wife held the light as I finessed the keepers into place.  This is not my first time with keepers so I knew what to expect. For a first timer, it will be pretty tricky because space is limited due to the rocker arms in the way. 
First I make sure I have the spring compressed enough. Then I place the keeper on the retainer. Then I get a knife or other thin tool to manipulate the keeper into position.  Sometimes, there's only enough space to get one keeper on easily. If this is the case, I install that keeper, and then rotate it to the side that is difficult to leave the easy side open.  Then I install the next keeper on that newly opened easy-side.

VOILA! You have new stem seals without removing the rocker arm assembly!

Nice write up"The Dog". This can easily save someone time and $$$ in replacing the entire head gasket.

Thanks bro, I hope it helps somebody, somewhere. 

Just finished my valve guide seals using "The Dogs"  method.  A bit tedious but successful.  Solved my smoking problem at startup.  Thanks for a great write up.

Cool DIY....

Thanks for sharing!

-Eric