I found this in HB Forum. http://www.homebrewtalk.com/showthread.php?t=447120
Rack to a fresh carboy leaving all the settled out yeast and sediment behind.
1/2 tsp potassium sorbate per gallon.
1/4 - 1/2 tsp TOTAL potassium metabisulfite for the batch.
Add and stir gently. Give the cider 4-7 days.
Then rack to a fresh carboy and sweeten. If planning on kegging/force carbing, you can rack to the keg instead of a carboy.
This my proven method for Sussreserve.
As to how much to sweeten, that is very subjective. You should pull a sample, taste, and add sweetener slowly to your desired level. Then scale up for the entire batch.
I sweeten with more cider, usually in a ratio close to 1:5 or 1:6 (1 gal cider to 5 gal hard cider).
Its very easy to make a sweet cider... OR a carbonated cider. But a bit tricky to make one that is BOTH. To do that you must sweeten, then artificially carb (force carb), or sweeten with something non-fermentable (like splenda or xylitol), and bottle carb with priming sugar.