Here's a few resources I've found over the years that have been helpful for learning myself, or teaching others:
A great tool for learning stalemate lines is the following, but it's also vital to be able to recognize and build lines yourself. (Wayback machine link since the actual site keeps going down) https://web.archive.org/web/20160510020009/http://diplom.org/Online/StalematesAtoY/visual.htm
Reading the full rulebook is a good idea if you haven't already.
http://www.wizards.com/avalonhill/rules/diplomacy_rulebook.pdf
Knowing where to read the Diplomacy Adjudicator Test Cases (DATC) is important if you have questions about how webDip adjudicates tricky scenarios or convoy paradoxes.
http://web.inter.nl.net/users/L.B.Kruijswijk/
A good document from which to learn basic tactics is this one: (Also Wayback Machine, as before)
https://web.archive.org/web/20160320155508/http://diplom.org/Zine/W1995A/Tactics/
Reading commentary and lectures from past Schools of War can be really helpful, as can be following the current one.
tinyurl.com/webDipSoW
Current SoW:
threadID=1515786Current Study Group:
threadID=1516035There was once a "Full Disclosure" game played by high-end players, where the press was formatted and published afterwards. It's useful to see how good players communicate.
tinyurl.com/webDipFullDisclosure1
This is a decent read on basic opening strategy:
https://en.wikibooks.org/wiki/Diplomacy/Opening_Principles