Writing for the Web:
Guidelines for MIT LibrariesSet the context
The "given --> new" principle
Start with what the user already knows or something that lets the user say "this applies to me," then give the new information.
Example: But if the titles you ordered through Tech Coop are only part of your planned book list, you will need to submit a complete list to Rotch Library’s reserves coordinator a full five weeks before the first day of classes.
(from Course reserves info for Faculty - Rotch Library)
Faculty members who have already submitted their complete list don't have to read the instruction. If it were in the other order everyone would have to read the instruction before finding out if it applies to them.
(In spoken language, people provide context).