Checking It Twice
Some tips for error-checking a budget.
Proofreading
Primarily, have someone else do it: you will miss your own mistakes most of the time. He suggests implementing a policy, or at least a practice, that everything is read by at least one person who hasn't seen the document before to catch errors. The more, the better.
Software Errors
Word processing spell-checkers miss many mistakes, and their grammar checkers are notoriously bad. Spreadsheet programs can make rounding errors when adding numbers (Excel may hide fractions, but it still calculates based on them) and there may be hidden errors (if your formula missed a cell in calculating a total, or highlighted the wrong column).
Cross-Checking
One suggestion: use a checksum technique to catch errors. You can add across and down columns to find errors. You can also use a separate cell that adds the same numbers together (don't' cut-and-past, re-create the equation).
Document Version Control
Documents often go through versions - and there is the danger of using the wrong version. One recommendation is to include the date of last edits in the file name as well as within the document's content.
Verifying Assumptions
If particular importance is getting one or more people to consider the assumptions made in a budget and weigh in on whether they feel they are accurate and appropriate. Since assumptions tend to b subjective, they are often questioned, and tend to be the cause of the most variances.