The present invention provides a method of forming a viscosified treatment fluid comprising: providing a treatment fluid that comprises water and a gelling agent; contacting the treatment fluid with a boronic acid crosslinking agent so as to form a crosslinked gelling agent, wherein the boronic acid crosslinking agent comprises a compound having the formula: ##STR00001## The present invention also provides methods of crosslinking gelling agent molecules, methods of treating a subterranean formation, methods of reusing viscosified treatment fluids, methods of fracturing subterranean formations, and methods of placing a gravel pack in subterranean formations. The present invention also provides boronic acid crosslinking agents and viscosified treatment fluids that comprise crosslinked gelling agents, the crosslinked gelling agent being formed from a reaction comprising a gelling agent and a boronic acid crosslinking agent.