When you sweeten, you need to use potassium sorbate to prevent fermentation from restarting. Sulfites don't kill yeast, and in fact wine yeast strains are very tolerant of sulfites (that's why winemakers use them). Sorbate doesn't kill yeast, either, but it inhibits yeast reproduction. so make sure that you don't have any lees on the bottom of the fermenter (which is mostly yeast) when your rack into the sorbate. Sorbate works better in the presence of sulfites, so you usually use both at that time.
Use 1/2 teaspoon of potassium sorbate and one campden tablet, per gallon of finished cider. Dissolve them together in 1/4 cup boiling water and stir well, to make sure it's well dissolved. Add that to the receiving carboy, and then rack the finished cider into it. Wait a couple of days, and then sweeten to taste.
You can sweeten with anything that sounds good. Honey, apple juice concentrate, sugar, splenda, etc. What I like to do is to pull a sample out and add various things and amounts until I get what I like. Then, I take a SG of the sample I like and sweeten the whole batch that amount. If I like it at 1.003, then I sweeten the whole batch to that amount. One thing- it does tend to taste a bit sweeter after being bottled for a while, so you might want to sweeten to just under where you want it to end up. For example, if you love it at 1.010, you may want to sweeten to 1.008.