A state of ultralow friction can also be achieved when a sharp tip slides over a flat surface and the applied load is below a certain threshold. Such a "superlubric" threshold depends on the tip-surface interaction and the stiffness of the materials in contact, as described by the Tomlinson model. The threshold can be significantly increased by exciting the sliding system at its resonance frequency, which suggests a practical way to limit wear in nanoelectromechanical systems.