I would also add acid blend to taste. Acid blend is a combination of tartaric, malic, and citric acid. Tartaric is found in grapes and malic in apples. I usually dissolve it in a little boiled water first. An accurate scale would make things easier. If you don't have one, I'd add about 1 level tsp per gallon if you think acidity is very low. As you get closer, I'd only add about 1/2 tsp per gallon.
Low acidity is common in much of the cider and juice sold in the US. If you make much cider, I would suggest investing in an acid test kit and an accurate scale and adjusting acidity prefermentation. You can get both for under $20. Here are two examples at Northern Brewer and Ebay.
Acidity is typically measured in grams per liter. If you test your cider or juice and it's at 6.0 grams per liter and you know you like your ciders around 8.0, then you just add 2 grams of acid per liter of juice. Simple.