Sounds like your springs are weak. If money is tight, go buy some coil rubber stiffners. It's a round coil made out of rubber and it goes inbetween the springs. It will stiffen your suspension and keep your car from sagging. Mr. Gasket and Spectre make them and they cost about $10 to $20 a piece at Autozone or Kragen. To keep them in place, buy some really long zip ties and cut the unused portion of the zip ties. You'll need about 5 zip ties for each coil.