Racing suit with valid SFI or FIA certification.
The Offical Rulebook
Innerwear with valid SFI or FIA certification (required for certain suits; check Lemons rules)
How Not To Fail
Lemons-legal helmet with valid Snell or FIA certification
Head/Neck Restraint (HNR) device with valid SFI or FIA certification.
Proper helmet anchors for HNR device (if sharing device, make sure all team members have the correct anchors).
Auto-racing gloves with valid SFI or FIA certification.
Auto-racing shoes with valid SFI or FIA certification.
Fire-resistant auto-racing socks.