Well, by now you'll have covered the material, so I think it is practice, practice and a bit more practice! Doing papers will help you pick up weak areas, identify time issues, and is a good way to practise specific techniques. For example the first round English paper is tight on timings, specifically due to the comprehensions. You could try a paper or two with your dc leaving these to last - we tried that, and realised skipping questions didn't work for ds, but at least it meant that he didn't try it as a strategy on the day!
Because it is holiday time there is a temptation to do a paper a day/2days/week. However in the actual test, your dc will be doing 2 papers with a relatively short break between. It is therefore better to practise doing 2 papers at a time, so that they get used to the stamina required. This is even more important when practicising the second round exams, especially the 50 minute writing exercise, when it is not uncommon to find that boys have run out of steam!