What do you need to clean teh freewheel?
How often should it get down?

I just cleaned mine after over a thousand miles of trail riding, and boy did it need it. I flushed it out with Gunk Lubricant several times, plus used a small brush to loosen the crud.

are you talking about a screw-on freewheel or modern freehub ?

I use and old tee shirt. I take the tee shirt and put a fold in it to get a double thickness, next, put the fold between each pair of cogs and slide it left to right repeatedly. This gets all the crud out and leaves the cogs looking nice and shiny. I'd be very wary of pouring degreaser between the cogs as this could work its way into the ratchet bit and maybe even your hub bearings.

With a cassette/freehub, why not simply undo the lockring and slide the cogs off the body for convenient, thorough bench cleaning, with no risk to the ratchet or hub bearings? For freewheels, the folded T-shirt technique described above works well.

Besides the folded Tee Shirt. or almost any kind of rag, Park makes a tool specifically for that purpose. (Rags are cheaper.)
If it is a FreeWheel and not a cassette, it is not a good idea to submerge it into a cleaning solution which will get into the bearings, dilute the grease and ruin the FW.
Stein makes a grease injector for FW's, but it ain't cheap either.

How can I re-grease it?

again, is this a screw on frewheel or a splined freehub. The latter has a lockring with an arrow and "tighten" or "unlock" or some such instructions.. The maintainance is quite different.

An even easier way to clean your freewheel is to spray some engine cleaner on it, then rinse it with a strong blast of the hose.

It certainly is NOT environmentally friendly, alas though, it does work well.

Neither is it bearings friendly !

I have always soaked mine in Kerosene for and hour or so, remove, shake it out, let it dry overnight, and then add #20 motor oil to both sides. So far, that's worked really well for me.

lj's thought that it will damage bearings is interesting though, I'll have to check that out.

Good Riding!

It seems to me that every method of cleaning that actually works is bad for some part of the bike or for the environment.

That's great Fubar! Truer words were never spoken...

Astra's probably correct that the solvent in the engine cleaner isn't good for the bearings.

I use foamy cleaner which does not SEEM to penetrate into the freewheel bearings. Guess you'd have to be inside the freewheel to know for sure.

Anyway, after weighing all points, I figure that the damage from the grit and grime on the freewheel is more likely to do more damage than the foamy cleaner.