Be very careful what you try to write off. And remeber, everything you buy out of state that you didn't pay taxes on, you must fill out a sepertate form and pay taxes for as the end user
Keep receipts for everything. Put them in a box each year and store them. Do not ever throw anything away. You will get all sorts of advise on how long to keep a receipt, but until you need them, you will never know.
And if/when you get audited, providing the auditor with a nice place to conduct his audit, will go a long way on making it a more pleasant experience.