Due to the nature of the sugars and the yeast ciders tend to ferment nearly all the way out so "stunning" the yeast with [ur=http://en.wikipedia.org/wiki/Potassium_metabisulfitel]potassium metabisulfite and I believe you can also use campden tabs- 1 per 5 gallons. Then you can add more sugars to "backsweeten" the cider.
Or you could use sugars that the yeast can;t ferment- maybe lactose? I've used splenda to lightly sweeten up a cider before and didn;t get any "synthetic" flavors.
Campden tabs are sulfites, so don't use both. And for best yeast control you'd need potassium sorbate too. Sulfites won't do it alone.
I've used splenda. It does sweeten, but doesn't add any mouthfeel, which I miss. Good for bottle conditioning though. I've tried some maltodextrin and found that it retained mouthfeel over time, but the sweetness fades over a few months. Don't use aspertame (nutrasweet or equal) as it hydrolyzes and stops tasting sweet within a few weeks. Haven't tried lactose.