improve eyesight for what? seeing spirits or just better vision? i've heard carrots help, but that won't give you perfect vision, just turn you orange. i don't really know of many that help you see spirits better. i came across a chant one day that i use a lot, but doesn't do much, i normally saying while meditating and i can here something in the room, though it never helps me see them, just focus in on where they are. another one i've never used helps for a couple hours. other than that, i've never come across one, and i wouldn't be sure how to make one.
if you do mean poor vision, i know of a couple vitamins and foods that help ones vision, but no 100% fixes.
Well statistically speaking your eyesight peaks at age 14, from there you can either take perfect care of your eyes or your eyesight will very slowly decrease over time. They have surgeries treatments to greatly improve your vision but besides that there's the idea of taking vitamins such a vitamin A which nourishes your cornea. Beyond that you could always try some basic reiki healing which includes controlling energy flow and charging. You can easily find tutorials on such methods, even here on SoM.
Bilberry tincture is very good for eyesight. Fighter pilots used to take this just before going off on their missions because it increases circulation to the eyes and causes clearer vision.
Increasing antioxidants in your diet such as eating blueberries a few times a week will also help.
There are actually a number of ways that one can improve their eyesight. Usually poor eyesight develops with poor visual habits - a lot of near point stress for example. There are a number of ways to improve your vision, including healthy eating, eye exercises and pinhole glasses.
Here's a great site that might be able to help you http://naturalvisionpinholeglasses.com