There are a number of ways to ensure crystals will stay on your nails.
1) Use a professional nail glue and follow the application instructions properly.
2) Make sure your nail is clean and free of any dirt or grease before gluing on the crystal.
3) Avoid doing anything that may scratch, rub or knock the stones off such as manual work, and wear rubber gloves to wash up.
) Make sure the crystal is placed flush against the nail, so there is no gap.