Stiffer, Stronger, Lower...Faster!
One of the first things enthusiasts do to increase the road manners of their ride is to install more aggressive springs.
Shorter, more aggressive springs achieve two goals: 1st - shorter springs decrease body roll and lower the cars center of gravity. The second benefit is that higher spring rates reduce suspension movement, with the hope of improving the wheel's contact patch with the road.
The downside is the eternal compromise: improved handling typically means a harsher ride. What is harsh to one person may not be harsh to the next. Though I used to think a "go-kart" suspension was the ultimate set-up, I find the older I get the less "harshness" I can take on my daily commute.
To help with the harshness issue, many spring manufacturers have "progressive" springs. What it really means is that the spring rate changes the more the spring is compressed. For example, a spring can be 10% softer than stock in its first inch of compression and then increase to 35% stiffer than stock as it is compressed farther. For daily driving, progressive springs will be more comfortable on the freeway and the provide the extra performace you want when cornering hard.
When installing new springs, be sure that the dampers (shocks) are correctly matched to the springs. Some spring manufactures design their springs to work with the factory dampers, while more aggressive spring rates require new dampers.
Also, when you lower the car, you automatically change the camber of your suspension (your wheels are tilted in). Though increased camber usually helps with grip at cornering limits, too much camber is a bad thing...just ask my wife when I wore out my Bridgestone S0-2's ($225 each) in about 3 months (asking to spend another $1,000 on tires that have no inner tread is not a wise thing). Therefore, be sure to get an alignment after installing new springs.