A cheap way and easy way, but not the safest or best way is to underinflate your tires. 28 PSI should be a safe low pressure for comfort, 35 should be fine for high pressure for best mileage. 32 is usually the compromise between comfort and mileage. Be advised that under 28 PSI will shorten the life of the tire, generate more heat increasing the chance of a blowout or the tire seperating from the rims.

Used Car lots do it to sell cars. I once bought a car from a used Ford Lot and for the first year I kept bringing my car in to check the shocks because it was too soft for what I thought it should be, it felt like a boat. Everytime they said it was fine. A year later I found that there was only 15 PSI in all four tires, when I inflated them back to 32 PSI it felt normal (Harsh and bouncy like a bus) ride. Even at 1/2 the recommended pressure, there was still no signs of underinflation - the tires did not wear more on the outside. I once had about 60 PSI (Super bouncy like an empty simi truck trailer clanging down the road) and no unusual tire wear, but I didn't have much traction, I did get about 45 MPG (10 MPG more than the sticker said I should get).