Other than doing a ton of pushups and building up that endurance...
Analyze yourself. What's giving out first?
Is it your biceps, your triceps, forearms, shoulders, abdominals?
Pinpoint the weak point in your form and work on that.
Work on any stabilizer muscles in your shoulders, do lateral and front raises, shoulder presses, chinups and pullups, crunches and cardio.
The biggest thing is simply going to be busting out those pushups, regardless of everything else you may just have to push yourself to exhaustion, give your arms a day or two to recover and do it all again. Eventually your arms will gain the endurance to do 80, 100, +++ pushups in a set.
Just be sure to give yourself a few days recovery before you have to show up and try to bust out more pushups at the ROTC otherwise it might not be pretty. :(
The cardio is important too, as the better your cardiovascular system is performing the more blood and nutrients you will get pumping to your arms, increasing load times and in this case staying power.
__________________
STATS:
Type: Ecto-Meso
Height: 6' 5"
Weight: 196lbs (July 6th) (Target: 225lb) (12lb lean mass gained since May 30)
BF: 12% (May 08) [</10% by October]
Nutrition: 4500 - 5200 Calories/day
Macros: 55C/25P/20F
Supplements:
+ MutantMass (gainer);
+ Dialene 4 (thermogenic / cutter)
Those things that hurt, instruct.
- Benjamin Franklin | 
