Great to know that you have started your preparations. Some tips that worked for me when I was preparing for the PST (YMMV if you have a different learning style):
1. The first and most important tip is to try out different practice tests! Not just the official three on McKinsey website (though that would be the best representation), but there are a couple of other free tests around e.g. MConsultingPrep, IGotAnOffer, and other sample questions e.g. GMAT! You can also practice with the BCG sample tests online too which also test similar problem solving skills. Don't just practice them once! You can work through each paper a couple of times (2-3), identify how you can improve, and try to go through the various practice tests in a rotational manner so that you can get the best out of each paper. A "safe" target score would be 22 and above out of 26 (don't expect to get that in the first practice!).
2. Also, it'll be good to practice doing mental math fast! Some great tools include Victor Cheng's math training tool, GMAT tests, and just general day-to-day practices. For example, you can incorporate mental math into your daily routines, like estimating how much it would cost if you bought a plate of chicken rice and coffee for each of your 26 classmates (within 5 to 10 seconds). The key is to be fast and fairly precise (+/- 5 to 10% margin, the closer the better). Just right before the test, you can try out a few simple calculations and times table in your mind (what is 7 x 15, 9 x 14) just to get your brain prepped for the number crunching later!
3. Lastly, given that you have a lot of time left, it is important to give yourself and your brain some time for rest, and not overwork yourself, so that you do not burn out right before the PST. You should plan a schedule that allows you to do that. It is very important to do this, because you'd want to feel confident, happy, and energetic when you step into the PST testing room, to perform your best :)