Recordkeeping that survives an audit

The rule (settled)

You must prove expenses. "You can't deduct amounts that you approximate or estimate."
Record expenses at or near the time. A weekly log counts as timely.
Receipts are required for all lodging and for any other expense of $75 or more. Below $75, a log entry with the amount, date, place, and business purpose is enough (Pub 463 ch. 5).
How long to keep records:
- Generally 3 years from filing.
- 6 years if income was underreported by more than 25%.
- 7 years for bad-debt or worthless-securities claims (Pub 583).

Home lease or mortgage statements, plus utilities during assignments
Rent paid by bank transfer, not cash. If renting from family, a written lease at fair rent.
Driver's license, voter registration, and vehicle registration at your tax-home address
Visits-home evidence: fuel receipts, boarding passes
Each contract and every extension (with the date agreed), plus the stipend breakdown
W-2s (check box 12 code L and code TT), 1099s, and your own 1099 income ledger

Lease or mortgage statements for your main home, kept during assignments
Utility, internet, and renter's/homeowner's insurance bills in your name at that address
Federal returns, bank, and professional license address = tax home
PRN or other work in the home area between contracts (pay stubs)
Agency per diem policy showing it's an accountable plan, plus your W-2 box 12 code L

Separate bank account and card for the 1099 business
Receipts or invoices: licenses, CEUs, certifications, insurance, software, equipment
Phone/internet business-use calculation
1099-NEC/1099-K forms, plus your own income ledger (1099s may not arrive for payments under $2,000)
Estimated-payment confirmations (IRS Direct Pay / Online Account)