I've done a few ginger beers as well. Unfortunately you'll never hit anything that tastes like good store bought ginger ale, but with alcohol unless you buy some from the store and add some alcohol.

My last batch was really gingery and crisp, but a little earthy, even after a year in the bottle. I would think that your best bets would be to either add some unfermentable sugars to the mix to retain the sweetness or stop the fermentation, add some sugar/honey and force carbonate. And of course, use good, fresh ginger.