Since I came to the side press through PTTP, I've never observed the USAWA (or Alan Calvert, as posted by
@Pavel Macek) locked knees requirement. One of the things I like about the side press is that it ISN'T as strictly defined as a military press or bent press (or maybe it is, but I don't feel constrained to do it "by the book," since I learned it from a different book).
In practice, I think of the side press as a press with an exaggerated "wedging" action (pressing yourself away from the weight). When doing multiple reps, I often raise and lower the weight in the same groove, without fully standing straight and squaring up in between. This may not be the textbook way of doing it, but it feels really good and natural to me.
Another unconventional variation I use with my side presses is starting the first rep from the top. I generally take the bar off a rack at shoulder height and push press it overhead using the off hand to assist (with a lighter weight, I will usually one arm snatch the bar overhead). Then I actively lower the bar into position for the first rep, building up the tension. I find this makes for a tighter, stronger starting position than taking the weight off the rack directly or shouldering it from the floor.
One big key I've discovered is to find the right gripping point on the bar. Don't center the middle of your hand. Center a point closer to the pinky side of your hand (thumb side closer the collar). Once you find the right balance point, the bar will be very stable and won't wave around. You can even press with an open hand -- the bar just sits there and goes up and down without rocking at all. It's actually a much more stable than a short bar or dumbbell.