Ways to: provide (context-sensitive) help in an Excel application

[lalbatros]

Many of my Excel applications come with a few pages of instructions.
Unfortunately, users don't read instructions usually.
The most important instructions are those that are the most difficult to explain: what kind of data are stored, what the user is expected to do, what will be calculated ...
Even comments inserted at the top of the worksheets are not efficient to provide help.

Maybe providing interactive help, hints and descriptions could help people learn how to use an applications more easily.

Any idea about methods, tools, examples, best practices for providing help in Excel applications?

[WinteE]

My experience with manuals, Less = More.

People don't like to read a lot, so you have to keep it short.