Weak coil springs are generally the result of hard usage, bad engineering from the factory or the use of inferior replacement springs. It is a misconception that a car's age alone contributes to the resiliency of the coil springs, since the metal does not significantly degrade over time.
Soft springs are caused by repeated work loading and unloading, and by the gradual heat tempering that occurs from suspension cycling. The repeated expansion and compression of the metal molecules during usage creates heat; and it is this heat that slowly lowers the spring rate and resiliency of the material. Since this mimics the original maker's practice of heating and slowly cooling the spring to temper it, the overall effect increases the metal's ability to return to shape but decreases its strength and ability to bear a load.
Simply put, the best way to fix worn-out springs is to replace them. If the vehicle is not particularly old or worn, it is possible that the manufacturer used softer springs than required to suspend the vehicle. As such, you might consider using thicker and stronger coil springs from an aftermarket manufacturer to increase the overall durability. However, be aware that this will degrade your vehicle's ride quality over rough surfaces yet greatly increase the car's cornering prowess and high-speed stability. Eibach makes excellent springs for practically every vehicle on the planet, and these come as original equipment in many cars.
If you were so inclined, you could actually revive the stock springs by reversing the process that softened them. But be forewarned: Although this approach is a great deal cheaper than buying new springs, it requires time and a quick and experienced hand with an oxyacetylene torch. Done incorrectly, it can damage the springs or over-stiffen them. This can result in inconsistent and possibly dangerous handling.
Using a torch with a medium blue-orange flame, heat the coil spring as evenly as possible to just below red-hot. Keep the spring just out of reach of the frame; otherwise you risk creating hot spots. This is best done with two people using two torches: one working on the top half of the coil, and the other on the bottom.
The key is to heat the metal to black-hot as evenly as possible. When the metal is sufficiently heated, dunk the entire spring into a bucket of boiling water. Lower the spring over a period of 10-15 seconds into the water, being mindful of the hot steam. Slowly lowering the spring will heat-shock the lowest portion into a higher level of hardness, while giving the upper portion a little more time to cool. This creates a more "progressive" spring than simply quenching the whole thing, and allows for a more livable ride with better handling at the limit.
This method of spring refreshment has been time tested. It is a cheap and viable repair, though not generally advised.
