These cars have quite a bit of body flex. When the top of the strut towers flex, it translates into a bit of slop in the suspension. Luckily, this can be cured with the addition of a strut brace. Basically, it's just a bar that has each end mounted to the top of the two strut towers. This prevents the struts from flexing in relation to each other.
Front 
Several vendors offer a bar for the front, but most of them just resell the same bar made by Jamex but sold (what is left) by ForceFedRacing. This particular bar mounts by either welding or bolting an included bracket to the top of the strut tower in the engine bay. For some mounting tips check out Huw Powell's page. The bar itself is actually two pieces that bolt to the brackets, with a turnbuckle between them to adjust the fit. The bar is shaped to avoid interference with the battery, but you may have to relocate some pieces of the cruise control. Jamex no longer sells direct, so you'll have to find a vendor that carries their stuff. Check around, because prices vary greatly. For reference, the direct price from Jamex used to be just under $100. The P/N is 310000.
Rear
The Coupe probably benefits from a rear strut bar more than the sedan, because the 90Q has the gas tank mounted between the towers, and the rear seats don't fold down. There used to be a specific rear bar available, but it has been discontinued. Some people have been able to mount a generic bar in there, though. If you get one, make sure it is easily removable, because otherwise it will limit your cargo carrying capacity (Coupe especially). The Jamex part pictured below is P/N 311001.
