The people I was associated with were either sales reps or publishers. From what I could see, they were definitely under a lot of pressure to look good and take their advertisers (people like me at the time) to the swankiest places in the city. They were responsible for making a lot of sales and were required to write a lot of proposals, often with very short notice. It seemed like a very high pressure job but since their salaries were mainly based on commission, it was in their best interests to really work hard. In a large city where I work, this was a good 7 years ago, but at that time the top salespeople were making well over six figures. It is definitely the type of job for those who thrive on a fast-paced environment.
If you're more at an entry level, you would probably have to start out as an assistant and then work yourself into a sales position. Of course, it would depend on the publication and whether or not you're in a large sales region. If you're interested in a specific magazine, look in the latest issue and see where the sales offices are (this info will be near the front and in small print). You may be able to call the office directly and see what their hiring policies are. Good luck!