Optimisation

1) Arrange your worksheets in alphabetical order with the sheets containing cells that are referred to a lot being at the start of the alphabet and those that contain a lot of formulas using those cells should be last.
2) If you are including user defined functions in your formulas, then try to put them at the end of the formula. For example instead of having "=UDF() + B2" use "=B2 + UDF()".
3) Any cell values which a user defined function might refer to are obtained at the top of the function and not after a lot of initial calculations.
4) MSDN Article - Performance and Limit Improvements in Excel 2010 - msdn.microsoft.com/en-us/library/ff700514.aspx
5) MSDN Article - Improving Performance in Excel 2007 - msdn.microsoft.com/en-us/library/aa730921.aspx