Ross Hunt, it will come with practice. One thing that helped me maintain a tight back when I was just starting was pulls to the knee. Loading is generally medium to medium heavy (Say you can clean 130 then you would do a lot of these in the 90-110 range). Just get down there set up nice and tight, retract the shoulder blades and get set to pull.
Now making sure you keep the same angle between your hips and back, just push back at the knee until the bar is about at the bottom to middle or your knee cap. Hold for a 1 or 2 count and reset for the next rep. We would do those in sets of 3 sometimes 5s with lighter weight.
Or once you hold for the 2 count you can continue on into a high pull. Mostly just pull to knee and hold though. What it should really look like is your shoulders are nice and far over the bar, then you start pushing those knees back, the bar should travel a pretty much vertical path (dont pull it back into you) so that when you hit the knee your shins should be abuot vertical with a slight angle from the knee up to the butt.
This will help you initiate with your hamstrings and use the low back to stabilize the first part, it will also help with keeping nice and tight and arched. If you have trouble with this you might want to work on ham exercises that involve hip flexion, we would do hypers with those giant rubber bands (the ones the powerlifters use for bench and such) and really accelerate at the top, step ups onto 18" or so box and so on. Take care, Adam.