Before You Buy

Check for a CPSC sticker (US) or equivalent safety markAny bike helmet sold in the US must meet the CPSC standard; look for the certification sticker inside the helmet. In the EU/UK look for CE EN 1078.
Measure head circumference with a soft tape, about 1 inch above the eyebrowsMost brands' size charts are far more reliable than trying helmets by eye.
Pick the helmet type for the riding you actually do (road, commuter, MTB, BMX/skate)Full-face and skate-style helmets meet different standards (like ASTM F1952 for downhill) than standard road/commuter helmets.
Favor a helmet with MIPS or an equivalent rotational-impact layer if riding in traffic oftenDesigned to reduce the twisting forces on the brain in an angled hit.
Get a kids'-specific helmet, not an adult one sized down if buying for a childThe foam density and coverage are different for smaller, lighter heads.
Never buy a used or secondhand helmet unless you're certain of its full historyYou can't see hairline cracks or crushed foam from a past impact, and there's no way to verify it wasn't crashed.

Fitting It Properly

Sit the helmet level on your head, about two finger-widths above your eyebrowsNot tipped back on the crown, which exposes the forehead.
Adjust the rear fit dial or pads until the helmet is snug all around with no gaps or pressure points
Buckle the chin strap and check the "V" of the side straps sits just below and in front of each ear
Run the two-finger test: only two fingers should fit between the strap and your chin when buckled
Open your mouth wide — the helmet should pull down noticeably on your headIf it doesn't move, the strap is too loose — a loose strap means the helmet can come off entirely in a fall.
Shake your head side to side and front to backThe helmet shouldn't shift or slide over your eyes.
Re-check fit every few months for growing kidsAn outgrown helmet no longer protects properly and needs replacing, not just re-adjusting.

Get padded gloves if you ride regularlyThey cushion the small bones in your hands in a fall and improve grip.
Add knee and elbow pads for mountain biking, BMX, or learning to rideMainly worth it for higher-fall-risk riding or beginners still finding their balance.
Choose bright or reflective outer layers for any riding near trafficVisibility matters as much as padding for avoiding a collision in the first place.
Fit a white front light and red rear lightOften legally required after dark in most jurisdictions, check your local road rules for exact requirements.
Add reflective tape or ankle bands if you commuteLeg movement catches a driver's eye more than a static reflector.
Get a properly rated motorcycle-style helmet instead if you ride an e-bike or moped capable of higher speeds if riding an ebike or mopedStandard bike helmets aren't rated for those impact speeds.

Replace the helmet after any crash or hard impact, even if it looks fineThe foam liner compresses on impact to absorb energy and doesn't spring back, so it won't protect you the same way twice.
Replace a helmet after 5–10 years even without a crash, per most manufacturers' guidanceThe foam and shell can degrade with age, UV exposure, and temperature swings.
Store the helmet away from direct heat and sun (not on a car dashboard or trunk)Heat can warp the shell and degrade the foam over time.
Wipe pads and straps clean with mild soap and waterAvoid solvents, which can break down the foam or shell material.
Inspect before each season for cracks, frayed straps, or a loose buckleReplace immediately if you find any.
Re-fit check after a haircut, weight change, or if it's been passed to a different riderA fit that was snug before may now be loose.