For keeping the valves seated and not dropping, I like to put some soft nylon rope in the cylinder, but you can also use a compression tester hose to fill the cylinder with air.
as for a tool, there's one you can get for about 40 bucks from snap-on, it's an AWESOME little tool....
works frickin' awesome, no need for levers and such.
It's a Blue-Point GA317, Small Valve Keeper Remover and Installer.
here's another one:
here's a link to the PDF:
Linky
here's another thread where the first image is also posted. Actually a write-up on replacing the valve seals on a toyota 4AGE engine (not my engine, although I've got one):
AEU86 forum linky
$39.05 plus shipping...
http://buy1.snapon.com/catalog/item.asp?store=snapon-store&item_ID=78777&group_ID=1578 Maybe less, maybe more, if you can find a Snap-on truck around.
I started to make my own tool, to do the lever-action style of removal, but this one just works SO freakin' good. install AND uninstall, hassle-free!
I just got done using it on my 4AGE last weekend, upgrading my valve springs for the new cams I'm putting in. No need to use a hammer like the pic on the box shows. The valve springs on DOHC engines are generally WAY softer than those on pushrod single-cam engines. I think the seat pressure on the Toyota engine I just did was ~30lbs. About 45 minutes, and I had all 16 valvesprings changed out.
--sarge