You need to discuss different possible segments of the populations to which you might market. Throw everything out on the table, literally. Grab a big pile of paper and cut it into 2 inch squares, then hand a stack to each person in the brainstorming sessions. Spend 5 minutes writing down different aspects of possible groups, one aspect per piece of paper. Mix all the scraps of paper together after the time is up and post them on a board or lay them out on a table. Discuss each word as well as different combinations of words. If nothings else, this exercise is a great way to get the creative juices flowing.
There are thousands of ways to get started, but once you open the flood gates, you and your team will enjoy a lot more success. Helping to target your brains will help you to focus on the immediate task at hand. Of course, there is more that must be done to pick a target. You need to look at hard data, including growth over that past six quarters for different markets of interest, as well as various demographic factors. Remember that you're trying to "clean house," so don't slip back into the same old targeting patterns, but don't be discouraged if you end up with a target that looks similar to the one you had before. It should really be a more FOCUSSED target than you had previously.