That will depend entirely on what branch or type of advertising you want to be involved in. Above all, you need to have a good sense of pop culture and know what the public finds interesting and amusing at any given moment and be able to create within those parameters, or at least be able to successfully parody them.
Writing skills, including an expert command of language are essential. Some graphic art skills are helpful, even if you aren't specifically aiming to be an illustrator, but they may not be necessary, depending on your specific job or department. You don't have to be gregarious and outgoing, but a good sense of diplomacy helps, as it will occasionally be necessary to deflect a bad idea without injuring someone's ego.
A healthy sense of humor and pride in personal accomplishment, without a narcissistic ego will keep you working and respected. Know your abilities and your limits and be proud of your successes, but don't fall into the trap of letting sycophants blow sunshine up your skirt and lull you into believing you're better than you are. The minute you start believing everything you do is great, that's the minute you stop improving.
Last edited by Willard on 29 Jul 2013, 4:54 pm, edited 1 time in total.