To make the most of your advertising dollars, use a "two-pronged attack," says Rao Unnava, associate professor of marketing at Ohio State University's Fisher College of Business. His research shows it's best to run a few ads in the same medium as your competition, while also advertising in a medium your competition ignores.
In a controlled setting, Unnava measured students' recall a day after they had read and heard a series of ads. He found the effectiveness of advertising drops substantially when there is "competitive interference." But if the advertising is in a different medium than the competition, its effectiveness is preserved. He suggests undermining your competitors ads in their chosen medium, while also seeking out virgin soil for your ads.
His research, published in the Journal of Marketing Research, found using one radio and one print ad was much more effective than two print or two radio ads. It was also more effective than launching a media blitz.