Keeping track of legitimate business expenses is certainly important. I have a separate checking account with separate checks and check cards for unquestionable business expenses. My annual marketing budget would probably make you faint, so rest assured that is written off. I have a car that I exclusively use for business purposes because it’s wrapped as one big mobile billboard. So I write off the whole car and everything about it. Before I had that car I never kept track of the mileage because it’s not worth the hassle to me. I didn’t keep track of gas then but now I absolutely would if I were you.
After it’s all said and done, taxes for me are pretty simple. The expenses are all kept in one place, Me and my business are kept very separate in terms of money, and I don’t reach for questionable deductions.