We have pictures of the trenches, as well as a drawing with distance from permanent objects for our utilities on the farm.  It helps a lot when repairing or adding new utilities.

The house was built in 1902, and the owner had a water tank in the attic that was filled with a windmill close to the house.  He had three-quarter pipe running all over the homestead that we still run into sometimes.  No map of all that work, but we do have reference to the other stuff we have ran to replace lines.

I still, if I had it to do over, would run a tracer line as someone else mentioned, so things would be easier to find.  At the time we dug all those utilities in, I had never heard of such a thing.