There isn't a way to revive a
unit (non-hero) directly. Once it is dead, it can't be revived without recreation. (unless maybe you have resurrection or something, but I don't know if that just creates clones or what) As far as unit recycling goes, I think they have damage detection to create "simulated deaths" at 1 hp. Play death animation and decay a bit and then off to heaven to be recycled.
As far as systems go, this is the only one for unit recycling:
http://www.wc3c.net/showthread.php?t=101600
Unit recycling has a few complex problems. You have to worry about things such as explosive deaths (mortar, demolishers, etc.), you have to worry about the "heaven" units triggering events or still being in groups, you have to worry about the facing they have once they are revived (since it is not instant), and there are a few other things that are probably annoying to fix.
All-in-all, unit recycling requires a lot of dedication and scripting to make it right. It does make a difference in maps and the memory they use, but overall it is a pain and most players won't really notice the difference. Generally, dummy recycling should be enough. There are a few systems for dummy recycling on this site, maybe those might be of interest to you.
I'm not trying to bum you out or anything, I'm just saying that it is pretty hard, and there isn't a real clear way to do it, it depends on the map.
