Trust me--everyone has to go through the crap jobs before they're able to kick off their actual career. But it's perserverance that seperates the deadbeats from people who are actually able to support themselves. So stick with it!
I worked in a grocery store for several years, so yeah, i know what it's like dealing with all sorts of customers (Everyone needs food, right? Even the assholes). And these people weren't just snobby--some of them were downright vicious. Like there was this one guy (we shall call him Bag Man) who, as soon as he would come into your line, would shoo away your bagger (which sucks when you've got seven people in your line, and you want to keep your bagger to expedite the process), do it himself, brag about how fast he bags, loudly talk about his "liberal arts education" and find as many things as he could to belittle the cashiers about.
For instance, when he went through my friend's line, he asked if he was going to Irish Fest that coming weekend. The cashier said, "No, I would, but I have to work." And Bag Man said, "Oh. You come from one of those poooor families, doooon't you." Seriously. He said that.
There was this other time when he came through my line, and I had tendonitis in my wrist, so I was wearing a brace. I could tell he was just winding up for some smartass remark, because I had dealt with him several times in the past--so I was prepared for him saying something. He looked at me and said, "Soooooo. What did you do to your wrist, hmmm?" So I looked up at the bastard and said, "I broke the jaw of a customer who was disrespecting me." He didn't say a thing.
I don't normally condone violence, but I felt reeeeeally good for the rest of the day. :D
Sorry for rambling, but I guess my point is just to stick it out. And let a customer have it every now and then, if they deserve it. ;D