We make egg creams at our deli. Here's one I made recently:
We used canned seltzer/sparkling water. It only works for a fresh can. Here's what I do.
* Fill chilled glass about 1/4 to 1/3 full with milk.
* Pour seltzer from can held about six inches above into glass until head is about 1/4 inch from the top
* Squirt chocolate syrup (we used Dagoba, which is actual chocolate, unlike U-Bet, which frankly, sucks; though if you're looking for it, I've even seen it at Wal-Mart around here) into glass, probably about 3 or 4 tablespoons worth, but it doesn't have to be exact; do it by feel
* Insert a spoon to the bottom of the glass and make rapid, short up and down movements. This is the key. You don't want to stir. Move the spoon around the bottom of the glass while you do this. The more you stir, the less head you'll have on your egg cream.