These include small diameter (eg. 1/8 in) length of rope, which is wrapped around the boom and through the clew 2 or 3 times, and then tied it off. Be careful not to tie the clew off too tight, otherwise, it will not slip along the boom when adjusting the outhaul.
Other designs consist of a strap (eg. velcro) that holds the clew close to the boom but allows it to easily slide along the boom as the outhaul is adjusted.
Having the clew secured close to the boom is important to ensure best sail control and trim.