Purdue online writing lab give help on revising for conciseness and clarity. This is very important when I do my instructions because for my instructions to effective people must understand what I am say and quickly.
I think this is interesting coming from people who develop and release email software what they find appropriate and deem 'good' to do in a business related email setting.
This was my favorite website out of all the one that I came across. This list was very well put together and it is presented in a very professional manner.
The best tip given is to not use email to avoid personal contact. Face to face time with a person is valuable and most of the time allows for better communication understanding.