Making your projects out of sturdy, treated wood is more attractive and will last longer than aluminium or PVC. It's no different with clothes line poles. Done right, they will be in the ground for years to come and the only thing you will need to replace every few years will be the rope. Depending on where you live, you can use your clothesline year round saving on those high energy bills.
Mark each one of your poles about 2 inches from the top and centre from side to side, drill a 1/4 hole all the way through.
Take your post hole digger and dig down 4 feet into the ground. Dig two sets of holes, poles twenty feet apart and sets at least four feet apart. Place your poles in the holes.
Mix your cement and pour into the holes with the posts, two or three feet up. Make sure your poles are standing up straight, as when the cement is dry you will not be able to straighten them. Let the cement dry and fill in the rest of the hole with dirt.
Pull your rope through the hole you drilled at the top of the pole to the other side and tie it in a knot. Go back to the first pole and cut the rope with enough left over to tie a knot at this end. Keep the rope tight as the wet laundry will pull it down and stretch it some. Make sure your knot is tight and won't come undone once the laundry is on the line. Repeat this step with the other set of poles. Now you have 40 feet of clothesline to use and very sturdy poles.
Give yourself more line by setting your poles farther apart then cut the outside of the pole, an inch from the top, two inches in and four inches down. Insert a treated 2 x 4 and nail into your pole. You can drill holes in the 2 x 4, pull through some more rope and have more lines. Just remember that wet laundry is very heavy, so don't make too many lines. Individual poles are much stronger.
Take a lighter and burn your knot. Have a glass of water with you in case it get too hot and catches fire. If you just melt the nylon coating on the knot it will fuse it together and won't come undone.
Don't make your poles too far apart as the line will sag and the clothes in the middle will end up touching the ground. Don't place your clothesline under trees. The birds in the trees will make a mess out of your clean laundry and you will end up washing it over.