I've had the greatest success by using a compressor that is internal to the spring coil. It is inserted from the bottom and hooks around the spring wire in two spots, 180 degrees apart, near the top of the coil and in two spots 180 degrees apart near the bottom of the coil. The screw is tightened from the bottom and the coil is compressed until it will fit into the spring pockets. If you've already tried this approach and found that it is difficult to remove the compressor once the spring is installed, try completely disassembling the compressor mechanism by threading the screw out of the hook mechanisms and removing it one piece at a time. Occasionally, if there is no engine the car, some of the parts may be manuevered out between the coils. This process may sound laborious but it has worked for me many times.