Lower the barriers to entry. Simplify the survey, make it fun and engaging to enter, take feedback in a place where people have nothing better to do.

Unfortunately SurveyMonkey isn't best suited to gathering consumer feedback. The best examples I've seen have been Innocent Smoothies (throw your empty bottles in one trash can or the other to give your "response"), and the taxis here in Dubai, which have a 5 question electronic survey on the back of the drivers' seats. Go for the win and put the survey in a public urinal and have people "respond" by peeing in one direction or another.