There is no write-off for per diem, it is by its nature tax free.
If you get less per-diem than the government allows (dunno the figure, around $30-40/day) then you can take a deduction for the difference between the government transportation worker per-diem allowance and what you got for your days on the road.
In the vast, vast majority of cases, folks get pretty close to exactly the government per-diem rate so you get to deduct zero.
Another way to do it is to add up all of your expenses, and if they are more than the per-diem you received, deduct the difference. Better have receipts.
Every year someone yaps about taking a deduction for every hour they're away from home and how they get a huge refund. Yeah, until you get audited. If you get per-diem it's really unlikely that there is any deduction available at all. I've never met anyone who, other than cheating (and not realizing it) actually got a significant deduction.
Here's how it works: the government, knowing that transportation workers have huge recordkeeping challenges, offers a way to simply use a per-day rate. Most airlines pay around this rate, so it's a wash--you get tax-free money the government says, "don't worry about keeping actual records" and there's no reporting requirements at all at tax time.
Some airlines pay more per-diem than the government rate--then the extra is taxable income and you get withheld for the extra.
Unless you have incredibly high expenses on the road, it is unlikely that there is any tax issue one way or the other for you to bother about.
Now, I predict someone coming on here and explaining that their accountant figured out every city they spent more than two hours in, and took the full-day per-diem rate for each of those two hours, then used this staggering high number by subtracting actualy per-diem paid and got a big refund.
Uh huh. The IRS knows how many hours there are in a day. Also, usually these accountants use the higher governement per-diem rate that includes hotel costs, however, unless you pay the hotel fee yourself, you don't get to deduct it.
So yes, there are guys that scam the system and take huge per-diem deductions, but if/when they get audited, the IRS sees the "per-diem" section on their payroll records and figures out that they're double dipping. It comes in already free of tax, end of story.
Remember, most filings don't get audited, so when someone gives you advice about being aggressive (read: cheating) and getting away with it, it doesn't mean that their method is approved, it just means that they haven't had to sit down across a table from an IRS agent yet.
I've actually done the math by counting up the days I was on the road, multiplying that by the transportation worker rate, then subtracting that from my per-diem earnings. It's always been a wash and hasn't helped at tax time.