It may be that your erector musdlces around your spine may not be strong enough after the layoff. Perhaps concentrate on barbell (not dumbell) good mornings using a weight that you can comfortably handle for about 10 good reps. Make sure you form is superb throughout the lift (even in squats of course) to really make you body learn to keep your back arched. (Perfect practice makes perfect.)
Also, “blocking your midsection” means to take in a deep breath into your belly and hold it tight (just as if you’re trying to push out another poop on the toilet) in order to keep your intra-abdominal pressure high. Don’t rely on weight belts for this.
See Paul Chek’s “Backstrong and Beltless” for some interesting info: http://www.t-nation.com/readTopic.do?id=460511, http://www.t-nation.com/readTopic.do?id=460512, and http://www.t-nation.com/readTopic.do?id=460368