Worn springs contribute to tire wear and misalignment by altering your car's ride height. Your car is equipped with either coil springs or flat leaf springs (these are typically found only in the rear). Use caution when servicing coil springs, since the Service Manual will often recommend you use a spring compressor.
In rear-wheel-drive cars, support the lower control arm securely on a jackstand. DO NOT support the weight of a car on just the auto jacks while working underneath the vehicle. Apply the spring compressor and attach two spring clips (These will acts as an important reference point later on) and detach the sway bar pin.
Remove the old spring, stand it next to the new spring. Mark the matching spring clip marks, and attach clips here. These will help you set the spacing properly. Add spring clips to the new spring, reinstall it and then reassemble the lower control arm. Using the spring compressor, remove the spring clips when the new spring is aligned correctly.
Rear leaf-springs are sandwiched together by bolts which can shear off under heavy loads. Be sure to use replacement bolts intended for rear springs, built especially from very tough steel. Rear leaves are usually attached by two U-bolts to the frame.
Lift the car on jackstands and look for the two bolts, one at either side of the spring. Support the axle with jack stands before unbolting the leaf springs, then slide the old spring out and then re-insert the new leaves in. If the U-bolts are badly corroded, you may need to hacksaw or torch them off, and then replace them.
A spring repair shop might be able to re-arch a badly sagged rear spring to its original contour. The spring would not be suitable for this repair if they find any cracks in the center. If so, the spring shop often can make and install a new leaf inexpensively. They'll also have to install the correct anti-squeak spacers between the leaves.
If you are concerned about the correct ride height for your car, manufacturers have published charts of both front and rear specifications for both the garage and for the vehicle's service manual.