Experts behind the Practical Tax Loose Leaf Service say you must meet the following requirements to qualify for a deduction of expenditure and losses:
You must carry on a trade.
You must derive income, or show a probability that you'll derive income in the near future, when you carry on that trade.
The amount you claim must constitute an expense or a loss.
You must incur the expense or loss during the year of assessment.
You must incur the expense or loss in the production of the income.
You must lay out the expense or loss for the purposes of trade.
The expenditure or loss can't be of a capital nature.
The expenditure or loss mustn't be prohibited in terms of Section 23 of the Income Tax Act (or any other Section for that matter).
You can only claim expenses specifically provided for under sections of the Income Tax Act, i.e. you can only claim them as deductions once.
Well there you have it: Keep this checklist handy so you can use it anytime you're not sure if you qualify for a deduction.