I scored a 25 on QR... although I honestly didn't do much for this section, I thought Destroyer was good, I used a GRE book to practice some questions (note that the format of GRE math is different than on the DAT though), and Barron's DAT book. As everyone else has said, speed is key when it comes to this section - I've just always been good at doing calculations in my head without a calculator. Make sure you fully understand common questions like permutations, D=rxt problems, all the different variations of timed task problems (one person can complete a job in x amount of time, while another can do it in y amount of time... how long will it take them to complete the job together?), all of your trig functions (don't forget equations for sin (2x) and cos (2x) ... knowing these equations will get you fast and easy points on test day), etc.
Also StreetWolf (I think that's his name) is an animal when it comes to answering any math-related questions on this forum...