There's a couple design stations in the post-show area where you can practice the design without the time constraints. (You can go in there before riding if you want.) I often go in there before riding to make a design and put it on my band. (By the way, this is the one area I know of where information is specific to each individual magic band, not to the MDX account itself.) Then I just tinker around when I get to the queue area, making sure NOT to scan the band that has my good design on it.
One key thing when you get to the stage where you start adding accessories is that each choice shows you the point value of each option. You can just select the option that offers you the most points.
Note that colors and fancy paint jobs might look cool, but they use up time and don't contribute at all to your score.