I use a glass with...well...horizontal ridges. They're round, but the sides wave in and out. It looks like a stack of rings. I got them at IKEA a few years ago. I pour in two rings' worth of syrup, then three rings' worth of milk and stir thoroughly. This leaves the glass about a third full of chocolate milk. I then fill with seltzer. Total quantity is about 12-14 ounces, with about the top one-quarter to one-third being a head of foam (depending on how fresh the seltzer is). Seems to work.
Yes, the syrup does make a difference. The canonical syrup is Fox's u-Bet, and it's what you should use unless you just can't get it. A quick Google turned up several suppliers who sell over the net. Regular chocolate syrups, like Hershey's, are sweeter than Fox's, and this drink is sweet enough as it is. We've got a can of Hershey's Dark Chocolate that we'll try when we run out of Fox's.
I suspect the egg cream is an object of veneration for native New Yorkers because of its association with childhood there. I don't know of similar concoctions made in other places. It's not bad, but perhaps there's something about having one at Junior's or Katz's that doesn't quite translate to a kitchen in Fairmont.