if you're talking about mac's lashes, its simple. since mac's lashes are made of human hair, you can't get them wet. so you can't really "clean" them. to keep them from getting all icky, don't apply mascara directly to them. apply mascara to your lashes first, and them apply the false lashes. then use a lash comb and run it thru your lashes. this will make them blend better. after you take the lashes off, carefully peel off the old glue without damaging the band. then put them back in the container up against the curve. that helps them keep their shape. then you can reuse them.
how many times you can reuse them depends on how you care for them and the style. i love my #3s, and can get atleast 5 uses outta them, and with each use they look brand new. #36s however, i am lucky to get a second use outta them. because there are clustered, leaving blank space on the band, the band tends to kink easier (unlike the #3s, which are solid thoughout the entire band). once the band kinks, you either have to cut it at the kink, giving you two sets of half lashes, or you have to throw em out. wearing lashes with a kinked band looks really bad.
as for non-mac lashes, i don't know how to go about cleaning those.
