To clean eyelash extensions, use a gentle, oil-free lash cleanser. Moisten a lint-free applicator or a clean lash brush with the cleanser, then work cleanser across the top and underneath the lash line, avoiding tugging or pulling. Cleanse the lash line carefully, and rinse with water. Pat them dry with a lint-free cloth or let them air dry. Avoid using oil-based products, as they can weaken the adhesive and shorten the lifespan of the extensions.