I have a friend who has not used a commercial yeast in over two years. He has 7 different strains of wild yeast going. His basic process is to take about two tablespoons of yeast from his sanitized mason jar and put it into a starter. He then steps it up until he has enough to pitch. He hasn't had any problems. He's using yeast strains that are 3 to 4 years old in some cases.
I have a strain of wild yeast that I harvested two years ago in my wifes garden from a Russian Sage Bush (4 small flowers). I still use it on about a 1/3 of my brews. I have three other friends using it. A local craft brewer is experimenting with it.
The rules I follow are:
If it starts to turn dark, throw it away.
If it causes the mason far lid to deform, throw it away.
Every once in a while, harvest new from the primary of a brew, to get a fresh group going.
Use boiled and sterilized jars whenever putting yeast into a new jar.
Sanitize, sanitize, sanitize.