Depending on your time management, taking half a minute to clear your head can be useful. As can a "cheatsheet" written out during the test, or a better method of solving the problems(duh). 99% of the time, there's a "trick" to the problem. Sometimes taking a step back from the problem itself and identifying that trick makes all the difference. And it's quicker
One thing that you need to bear in mind is your mental state as well. Mental or physical fatigue are HUGE factors in testtaking ability. If your test center has a place to store your stuff like mine did, BRING FOOD. I myself am hypoglycemic, so this is partly specific to me. Eating a snack every break you can not only keeps your blood sugar up, but gives you something to concentrate on to clear your head.
It's tough, but try to stick to your testtaking status when you arent taking the real test as well. If you always practice when you're tired, your scores will be deflated from their baseline. This isn't going to be good for your confidence in the long run, despite the extra study time it gives. Might also want to look at placement of problems that you get wrong... If you think about it, STRINGS of right or wrong answers are more statistically unlikely than in a test with homogenously difficult problems; every problem you get wrong will be making the next problem easier, so you should not have a long string of right/wrong answers after the beginning of the test. If you've got strings of wrong answers, there is probably a correlation between problems- whether it be fatigue, or letting longer problems affect your stress level for subsequent questions.
As sad as this sounds... the biggest difference for me was food. Having a snack to concentrate on had a consistent effect on the volatility of my score.