I can't remember which Pentax you have, but since I've have 120-400mm for my K-5, I can give you some of what I do...

1. Shoot in bursts, especially for moving targets, but even if it is stationary burst shooting will help.

2. Use a tripod and get a head designed for long lens shooting. Wimberly makes some good ones, so does Arca Swiss.

3. Practice, practice, practice.

Now, rifle shooting techniques do actually help. When I was in my military days, I had a really good lesson on technique from a guy that taught shooting for the Ontario Provincial Police and these can work for photography:

1. Bring your elbows closer in to your body, letting your body help support. If you can, down on one knee resting the elbow of the arm holding the lens can help a lot. The strap can also add tension to steady your hands.

2. When you're about to shoot, take a deep stomach breath and take up the slack on the shutter. Let the breath out half way, hold and shoot. Don't hold your breath too long, you'll get hand shake.

3. When you shoot, squeeze the shutter, don't stab it. If you stab it, you'll move the lens.

4. Practice the technique on a subject that you don't care if you miss... It takes getting used to, but if you do it, it works.

Finally, don't just discard the mis-framed shots, sometimes they can be interesting: