The hardest part of the standing military press is not using your legs. Lock your knees out and squeeze your butt cheeks together to avoid this. Here are some more tips:
1) Use a relatively wide grip - about a thumb width from where the knurling starts on the barbell.
2) Use a relatively wide stance - feet wider than shoulder width.
3) Look up at the ceiling before initiating the lift - this helps get your chin and head out of the way so the barbell path goes straight up.
4) At the top of the lift, drop your chin, look straight ahead and put your head through the "window" formed by your arms, bringing your arms to your ears.
5) Except for the first rep, breath with the barbell in the overhead position. John explains that when the barbell is on your chest, it's hard to fill you lungs with air - it's much easier when the load is overhead. Take a deep breath, hold it, and execute the lift.