I think I can help. The classes I teach that get tested are 6th grade reading and 7th grade science.
In reading, our school has test prep books. I spend about 30 minutes a day, two to three days a week on the test prep book. My goal in this is to review concepts that were covered at the start of this year or in previous grades. I also am getting them used to the test format by giving a sample test and by reviewing test taking strategies. Our reading test has a writing component, but we have been reviewing that for a couple of months. I have also told the students about any websites I find that offer review for reading tests. Our state has a sample test that students can take on-line. Lastly, get your students to practice reading. I start my students at 20 to 30 minutes of free reading a night (unrelated to homework). By the time the tests come, they should be up to 45 to 50 minutes. The tests involve a lot of reading and they need to build up endurance.
In science. I spend two class periods a week using the test prep book. We teach Earth Science to our 7th graders, so mostly I am reviewing the other science topics that they cover in other grades (life science, physical science, etc). Again, I am giving a sample test to prepare them for the format.
Finally, if you are going to need to change your seating chart for the tests (i.e. students can't sit in groups or whatnot) then, if possible, I would do so now. The students will get used to the room arrangement and will be more comfortable come test time.
As you said, we don't want to swamp them. Most of the prep you do is in your everyday teaching. These are just some small things you can do to help them be prepared.