My wife and I failed to summit on our first attempt earlier this year and are making another attempt in a few weeks. This summer, I've watched 500+ Youtube videos on the mountain and obsessively studied acclimation, nutrition, pack weight reduction, just to name a few things. And despite all of this, I'm the last thing from an expert, so my tips below are things we've gathered either from our previous failure, information gathered here (tons of expert advice on these forums), and other places on the internet.
Altitude Acclimation: There's a Facebook group (https://www.facebook.com/groups/AltitudeAcclimatization/
) dedicated to the subject with tons of useful information. Some people can summit with absolutely no preparation and experience no effects of altitude. But you really don't know if you're one of those types until you're there which is a huge gamble. Study up on it and decide your gameplan. My wife and I are going the route of Diamox and two days of pre-acclimation this time, we just did a trial run of Diamox over the weekend to observe any side effects, we'd rather know what to expect before taking it for the first time on Whitney weekend. Since you're about a month out and if Diamox is something you will be considering, you may want to act sooner rather than later as our doctor had us go in for bloodwork before we could get prescribed, so it took a little time.
Gear: If you're staying multiple nights, I've seen a saying around here that every ounce starts to feel like a pound further up the hike. Be mindful of what you're carrying. I made that mistake last time bringing far too many luxury items or just unnecessary stuff.
Study the weather, start looking at https://www.mountain-forecast.com/peaks/Mount-Whitney/forecasts/4418
today and monitor it daily so you can see the trends. Also I keep a permanent browser tab on my PC and phone open on the live webcam, I start monitoring the times of day that the clouds roll in, very useful to know because popular opinion is to summit as early as possible in the day due to afternoon storms: http://www.mountwhitneyforum.com/webcam/camera1.jpg
Don't buy anything new and wear it for the first time going up Whitney. Shorts, shoes, shirts, socks should all be taken for a test hike or several, there's nothing worse than going up and finding those shorts rub your legs in an uncomfortable way and you're stuck.
The same goes with food. Try to avoid things you've never had before so that you aren't up there with stomach issues or feeling "off" because of what you're eating. Whether it's the pre-packaged foods, Nuun tablets, Gu energy chews, whatever it may be, it doesn't hurt to simulate using those same things before going out local hike or on a high-incline treadmill climb.
If anyone in your group has an issue with heights and you own any recent generation VR device, there's a few good Youtube 360 VR videos out there that let you experience what it's going to look like around some of the "sketchy" areas. HikingGuy.Com has a good VR video and TnTstrong has a full 6-hour top-to-bottom VR recording where you can see the entire trail from Peak to Portal. I personally have some issues with some of the sheer drops and seeing them in VR has helped me learn to focus forward and not be looking over the edges as often.
These are some of the things we learned from our mistakes last time. We did very little of this the first time and learned the hard way that when underestimating this mountain, it can be a rough go (in our defense, we're both 50+ and had never done a 14er before). Good luck on your September hike and looking forward to reading your trip report once you return!