Enlarge the print of your music. Do this by photocopying the page using a machine that has an enlargement setting (you will need oversized paper), or, if you are playing from new music that has been entered into a notation program (such as Finale or Sibelius), print a copy with larger font sizes.
Mark your music with color. Create a color code for yourself to help indicate the presence of small, detailed markings that are difficult to distinguish when you have limited eyesight. For example, the symbols for sharp and natural notes are quite similarly shaped, so you might mark (or have a colleague mark) the two symbols with different colored highlighter pens so that you don't accidentally confuse the two.
Read from an electronic source. Reading from a laptop screen will not only give you extra light, it will allow you to adjust the zoom on the image of music. Scan music with a computer scanner and save as an image file or PDF (avoid MIDI or notation scanning, as this process usually leaves too many errors in the music).
Use braille music. If you struggle with low enough vision that you can't read most music visually, look for music printed in braille music notation. Check out some of the links in the "resources" section below to get you started.
Get a raised image printer. For music that has not been converted into braille, scan and copy using a specialized printer that prints all lines with enough three-dimensional texture to be detectable through touch. Using this technology, you can convert any piece of music into something that you can read with your fingertips.