How are you doing untimed?
The best way to "speed" up is to get your untimed score up. Ideally you would want your untimed score to be 175+, however, this may not be that practical if you are taking the test in June. At the very least you should be able to get about 10-15 points above where you want to score. So if you want a 150, you are basically looking at a 165ish untimed. After all, if you cant answer the questions untimed you really won't be able to answer them timed. Note: When you are doing untimed practice make sure you are following a proven method (ie Kaplan, Powerscore, etc). Also - jot down specific reasons why each correct answer is 100% correct and, more importantly, why each incorrect answer is 100% incorrect.
Once you are in your target untimed range then you can start worrying about timing yourself.
Come test day you really don't want to have to "speed" up though, rather you want move at a heightened and focused pace. In many instances this may actually involve slowing down.
Finally, don't feel stressed over struggling with time. I've been studying for this thing for months and every now and then have to randomly guess on a handful of questions due to time constraints (and am scoring around the 98th percentile). The test is specifically designed so that the average taker will not come close to finishing on time.
So in sum - practice practice practice and more practice (smart practice that is).