Liz, since I'm a greeter, many people visit my profile page every day. I've included a few suggestions on the page including the following:
"Try to document and include the source of the document for each profile you create, especially when you get further back in your ancestry. [I'm still working on this one. Here are some of the research sites I use.]
Including this needed information will take time, but it's a good goal to work toward and will make much sturdier branches for the worldwide tree."
~*~
I was including this in a separate message on their profile pages when I confirmed people, but since I'd already given them a number of important links, I was afraid I'd scare more people off than help the site so I transferred it to my profile page where I know quite a few see it. Also, when people tell me they are new to genealogy, I encourage them to read the suggestions on my page. The above is just a baby step, but it gets newbies thinking about the importance of documentation and sourcing and, hopefully, quite a few will follow through.
What you wrote is really good:
"Please include sources for information that you add to WikiTree (uploading primary documentation or providing information that would allow others to find it is best, but at least some idea of where the information came from)."
My greeting is already quite long, but perhaps I could put a shortened form of the above in it.