As mentioned earlier, it's harder to get picked up the more senior you are. Being an MM1 gives you a better chance, as you're not as needed as ETs.
I was a LT (NUPOC) at prototype and I've done the selection boards before. Treat it as any job interview, as the process is about the same. Go to your local book store and read up on how to conduct job interviews. Your interview with the board and the CO will be like that. Prepare routinely asked questions and have preped answers. Study what an 'officer' is supposed to be, i.e. not just standing OOD or EOOW, but a tactician, warfighter, etc.... With the students, it was always clear who didn't research what he was signing on for. When asked why they want to be an officer, they'd say "I think EOOW is cool". Instant drop in rankings.
Questions that you should be 100% ready to answer in a heatbeat (for an E-6):
Why do you want to be an officer instead of a chief? (Get's rid of the 'leadership' aspect. Also, don't say better pay/retirement. While true, not what they are looking for).
Where do you see yourself in 7 years? (If you say "earn my degree, do my 5 years, then get out", expect not to be accepted).
What leadership experience do you have? (Sure, you've been a LPO, but what else? Lead any special programs? Fitness coordinator and directly raised the command PRT pass rate? Outside the Navy leadership?)
Be ready to expain any NJPs/bad evals. Don't say "my DH/CO was a dick". Tell how you overcame and achieved.
Why did you select ______ university to attend? Why that major?
Other than that, be professional and polite. Being an E-6 at prototype, if you do your board with a bunch of SEs, you probably have become buddy/buddy with a few of them. Make sure you put your sailor-talking-to-a-officer-in-the-military hat back on for 15 minutes. Iron your uniform, and make sure your paperwork is turned into on time.
Good luck on your STA-21 package. One of the few "good deals" in the Navy. Also good luck at NPTU, you'll need it. Any other questions, feel free to PM me.