Usually, a glove loses its stickiness when it gets dirty, so cleaning your gloves frequently is a great way to maintain stickiness. If they get filthy, roll up a wet wipe and rub it between your gloved palms to get the dirt out.
Also, storing them out of direct sunlight is a great way to prevent the sticky material from drying out.
There are also grip refreshing gels that can make your worn-out gloves sticky again.