You've already got the advantage here: you're an artist, and you're working with a sandwich artist. From one artist to another, game recognizes game. The key here is to remember that you are in charge. In charge of what goes in your sandwich, in charge of when you get your sandwich.
The most successful Subway customers, of course, are the ones who can't keep their hands off their sandwich. Join your artist in the sandwich assembling process. That sneeze guard is a suggestion. That sneeze guard is trying to intimidate you into staying on the "customer's" side of the partition. Are you a customer? Or are you a man?
If you want avocado, you'll get avocado. Avocado is a fruit; it cannot stand up to you. You are a human being, and a very powerful man. Avocado wants to be on your sandwich. It can't help itself. Your job is to make the avocado realize that you know where it belongs.
No one likes being ignored. This is as true for sandwiches as it is for you -- even more so.
Remember, great pickup artists aren't outcome-dependent. You can't win every battle, and you won't get every sandwich you reach for. But if you try often enough, and learn from your mistakes, you will improve your SMM (Sandwich Mind Mastery). There are several different schools of thought about the best initial approach, but we won't go into the relative merits of neuro-rotator programming versus the Dark Jugglist's Method here. The most important factor is confidence. Make a connection.
You may find yourself feeling awkward or even reluctant to cross the customer/employee barrier; this is normal. Your mind has an evolutionary circuit that leads to this "approach anxiety" that stems from the cavemen days when men were stoned to death or eaten by mastodons if they failed in their approach to leap across the counter. One way to overcome this fear is to introduce seemingly innocuous observations that appear to be compliments on first blush but are in fact designed to bring the sandwich and its attendant components down to your levels.
Everyone wants to be liked; everyone wants approval. No one likes being ignored. This is as true for sandwiches as it is for you -- even more so. Their whole reality is based on having power, on being desirable. Take that away, and their whole reality crumbles.
This isn't a technique that works well on your average sandwich: your basic turkey sub, your standard veggies and cheese. They're used to being put down; you don't need to put them down through value zingers. This is for the special little princess, the daily special, the one that's used to being coddled and complimented and adored by everyone in line. Here are a few possible options: