I faced the same problems as yourself when I first started on the 737-300.
I found that the following helped.
Seat Position: Make sure this is the same every time. I'm sure you've found that there is a numbered scale on the height adjust lever - find what works and stick with it. Read the FCTM which gives good advice on where your seat should be positioned to give you the correct sight picture.
During the approach, barring weather, I would imagine you'd be stable by 500 feet. If the automatics are in, when you click them out, you shouldn't need to make any adjustments as the aircraft was previously in trim. I've seen people who then start moving the control column quickly from side to side and fore/aft. These little movements have little effect on the aircraft and will most likely result in pilot induced oscillation and could cause you to become unstable.
At the point you click out, take a deep breath and relax. If you are flying with FD on, keep flying the ILS down. The biggest mistake is opting to go visual and then relaxing the controls and going high on the G/S. Make sure your scan is between the PFD and outside. Bring in the VSI as well to make sure you ROD is constant.
Next time you do an autoland, watch when the thrust levers retard. It's around the 30ft callout. You'll then see the aircraft flare for landing. If you can replicate this, you shouldn't go too far wrong.
I'm sure you'll get plenty of advice. The important thing is to find something which works, stick with it and once you're happy, you can start finessing things. Just remember, a Boeing landing isn't a greaser which you hardly feel.