As none (or very few) of you know, I work as a cashier at Meijer (I scan and bag the items the customer wants to purchase). For those who don't know what Meijer is it's a super store, think Wal-Mart, but with a worse working environment, and it came first. I've been there for about a year and a half now, so I've basically learned the ins and outs of working there. Pretty much every Customer service position is the same, so once you know what you can and can't do at one work place, you are set for the rest of your career in customer service.
Don't ever let management see that you are having a good time. Smiling and laughter, no matter how few customers are around, mean that you're not doing your job properly, we shall therefore find something constructive for you to do with your time (usually some type of cleaning).
Never expect your break on time. According to Union (I hate Unions) contract, we're supposed to get a fifteen minute break for every two hours of work. I've worked shifts where my first break wasn't until four hours into my shift. And then they have the nerve to try and send me on my lunch thirty minutes after I get back from that break.
Don't work any harder than you have to to keep from getting fired. Okay, this is a big one. It should be one of the first things you learn. No matter how hard you work, it won't matter. Promotions and hours given are based on seniority, not on how hard a worker you are.
Never date a co-worker. Yeah, this is another big one. I had to learn this the hard way (twice). It started out okay, but then I found out that rather than a sylph, I was dating a banshee.
Those are the biggest things I can tell you not to do in a customer service position.
Based on moral alone, if Meijer and France were to go to war with each other, France would actually win the war. They would then quickly surrender to the nearest person they found after the war to shake the unfamiliar (and thus very scary) feeling of victory.