Anyone who has gone to college or has sent a child to college is familiar with the sight of an old sofa on the porch of a student rental. Although the sofa is grubby and torn, most kids don't mind using it. That image may make you cool to the notion of using an old sofa on your own porch, but you can do a much better job with a sofa by protecting it from the elements and keeping it attractive. A sofa is one of the most comfortable pieces of furniture you own, so it's ideal for an afternoon or evening of front-porch relaxing.
Select a spot on the front porch that tends to stay dry during a rainfall. Have a tarp and tie-down straps handy, though, to throw over the couch when a bad summer storm hits. No matter how well covered your porch is, a bad storm can drench just about everything.
Move the old sofa out to the porch. Place the sofa in the dry spot that you picked. This is usually along a back wall.
Remove each of the cushions from the old sofa, and put them into zippered cushion covers. Zip each cover closed to keep the cushion dry even if the couch does get wet. Plastic cushion covers are widely available, but if you search you can also find covers in water-resistant fabric. These fabric covers are more comfortable and more attractive than the plastic alternatives.
Cover the rest of the sofa with a quality water-resistant slipcover. Make sure the slipcover fits tightly around the base, back and sides of the sofa.
Use throw pillows designed for outdoor use on your porch sofa. These vinyl coated throw pillows are available at department stores, garden centers and other places that sell patio furniture.