The best write-offs for your business are anything you CAN write off. If it's a business expense, it's deductible. You should even keep track of the expenses you're being reimbursed for. It's quite possible that your clients will report the amounts they reimburse you on a 1099 to the IRS, so you'll need to record that as income and then report the expenses along with the rest of your business expenses. This way, you'll be able to keep track of it in any case.
And basically, anything that's business related is deductible. Just make sure to keep proper documentation for everything, in case you get audited. Here's a link to an IRS page on this issue: Deducting Business Expenses.