It depends on the item(s) being held down but in general bungees are my preferred method with a bungee net being the most secure of all. Don’t buy cheap bungees at the home improvement store. Get high quality shock cord and fittings at a marine chandlery and make your own bungees. Best if one end is permanently fixed to the rack.

Cord and rope are the most unreliable. Web straps with cinch buckles can works well if the load has some compression like a tent or sleeping bag but if the web slips even a little your out of luck. Solid items like hard plastic cases tend to jiggle out over time even if the web doesn’t slip.
Lace and tie shoes through the rack. If bags and the like have a loop or handle pass the bungee or strap through it. Roll down dry bags can be clipped to the rack as back up security.