Will use a PR when doing overhead presses. Include doing quarter and half reps partials. Push presses (bar resting on the heels of the palm) give a more direct inline power surge with the wrist/forearms. With a regular grip (bar resting on the palms & thumbs wrapped around) the bar/weight is off center of the line of power.. Some heavy bencher's also use this style, bar resting on the heels of the hand. The jerk press is another superior version of the overhead press.
With standard pressing, don't want to lock the knees, but have a slight break in them, or else the small of the back gets undo pressure. For strict BB'ing, pre-exhaust front raises with BB presses. Even SS'ing up-right rows with overhead pressing.
For me, one of the better exercises for cleans (and Hi-pulls) is doing them with the knees on the floor (usually folder mat). Seems to develop the upper portion of the power pull quite well. Also pressing from this position.
The side press and sea-saws are my two favorite DB shoulder movements. Good Luck