I think I mentioned this before about the batteries coming out:
1. Make sure the slots are filed down. I mean really filed down! If your cells aren't over 0.5mm to 1mm above the bottom of the chassis, they are sitting too high. A large flat file on a chassis plate (with nothing attached) is the best way.
2. Make sure you aren't using some knackered old packs. I've seen sponsored drivers using old practice packs that have been twisted back and forth and are all bent out of shape, so I know plenty of normal drivers are doing this too. If you don't use the battery boxes and just toss the packs into your pit box, you're probably twisting the packs more than you think. Make sure the battery sits flat or very close to flat (check this by putting the pack on a cutting board or tweak board or kitchen counter or something and see how twisted up the pack is). Use a good soldering jig (like a Dean's jig) and some new battery bars if you're building up new packs for your car.
3. Make sure the battery brace is tightened down properly. If it's too loose the cells will come out anyway, no matter what you do. Put the foam on the bottom of the brace and tighten down the bolts so the body clips hold the pack tightly in the chassis. I don't use a battery brace, just tape and after a couple of tries I haven't had any problem with the batteries coming out since the first day.
Does anyone have a Pro 4 site yet with a collection of these tips? That way the guys on the forum can direct new owners to a single site with a bunch of tips, instead of repeating themselves.