This is more of a question for an accountant than a lawyer. The general rule is that all 'ordinary and necessary' business expenses are deductible. The IRS has published an article on this question, available online at
If you have specific questions, you should ask your accountant.
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In substance, I basically agree with Mr. Bodzin's answer. I'll just add that  if you aren't inclined to hire an accountant or a tax lawyer to parse over your specific expenses, you can buy business tax software that will walk you through the types of expenses that are deductible, and  while you're trying to figure out what's deductible and what probably isn't, be sure to save all of your receipts!
As mentioned above, business expenses that are "ordinary and necessary" are generally deductible. A good discussion of this standard is available at: http://taxguide.completetax.com/text/Q13_2512.asp.