S.W.
One thing that I think has a big effect on how kids do on standardized testing is stress. Today's public school systems have kids so stressed out about tests, especially TAKS, that the kids start to second-guess themselves, change answers, and doubt their abilities to do well. I think the best thing you can do is tell your daughters that no matter what the outcome of the test, you know that they did the best they could and they should be proud of themselves. I know that doesn't help if one of them has to repeat a grade, but maybe that's what is best if they truly aren't learning the material appropriate for their grade level. Also, if a child does well in their school work, but fails a test covering the same material, that could be the sign that they need help with test-taking skills and might even need a modified testing environment. Talk to your daughters' teachers if you're concerned and they can help refer you to a diagnostician who can help them with their test-taking skills.
On the day of the test (and even the night before), make sure they eat well (no skipping breakfast), get a good night's sleep, and repeat to them as often as you can - "I'm proud of you and I know you'll do the best that you can."