In order to break into commercials, an actor must have the opportunity to audition, and having an agent increases the chances of getting a casting call. Some agents specialize in commercial actors. Actors just starting out find that commercial agents are more willing to work with newcomers, as commercial agents are not as focused on past acting experience. Before searching for an agent, have a professional head shot ready to show. Once you have an agent, work with her to determine what kinds of calls you will and will not take.
Improv classes are a great way to learn spontaneity. They also teach methods to "sell" an audience in a very short period of time. Some acting schools offer courses in commercial acting, where actors learn to quickly present an image, such as dad, runner or college professor, rather than develop a character arc. A seminar on sales techniques can also be beneficial, not only for commercial auditions but also for personal development. Any course work taken looks great on a resume.
Commercial auditions require a different skill set than auditions for movies or television. Often, actors are not given scripts until the actual audition; other times, the actor is asked to improvise. Directors are looking for genuine, honest people who appear likable. Try to imagine that you are recommending the product to a close friend. Warmth, openness and a genuine quality will go a long way in the audition room.
Before the actor walks into an audition, the directors and other creators know the appearance and personality they're looking for to sell the product. Instead of trying to morph into a character, be true to who you are. The sincerity you convey will get you further than faking a personality you think the director wants.
Unlike some industries, dressing for success in the acting field does not necessarily mean wearing a suit and tie. Dress the way you think the character would. If the character is a professional businessperson, a suit might be appropriate, but a mom handing out peanut butter to kids is more likely to dress in jeans and a loose-fitting sweater. If no information is given about the character, wear clothing similar to that worn when using the product.