One of the areas where I have some difficulty with my visual impairment is finding the cursor on the screen. I love that OS X has a large cursor as one of the accessibility features. The large cursor setting can be found under System Preferences, Accessibility, Display in Mountain Lion and System Preferences, Universal Access, Mouse and Trackpad in other versions of OS X.
However, the large cursor is not always enough for me, especially as my vision gets worse. I would love it if there were mouse trails or some other visual indicator to help me more easily pick out the cursor when I move it.
I have used a number of cursor enhancements for the Mac in the past, including Mouse Locator and Mousepose. Right now I am trying Omnidazzle for this purpose and it is working rather well. Omnidazzle is a free cursor enhancement intended as a presentation tool to help the presenter highlight key information on the screen. By pressing a keyboard shortcut (usually Control + `) you can bring up effects such as a spotlight, highlighting the foreground window, and more. The one that I have found the most useful for me is the Bullseye one.
I have set this effect to bring up the bullseye when I shake the mouse. This is great, because whenever I lose the cursor the first thing I try to do it is move it around quickly to create some movement on the screen that I can pick up with my limited peripheral vision. With Omnidazzle (sounds like it was created by Snoop Dogg, doesn’t it), a large red, bullseye will come up around the already large cursor. You can change the color, but red is one I can easily pick out.
Omnidazzle is freezzle, so check it out and let me know if you find it helpful.
The Website of Luis Perez 
I have Usher’s Syndrome, which includes both tunnel vision and hearing loss. I tried Omnidazzle, and agree that the Bullseye is the most useful choice. I have a trackpad for my iMac instead of a mouse, and tried ‘Shake’. It does work. I found it worked best for me if I reduced the number of shakes to the minimum of 4. It is necessary to move your finger back and forth over least an inch quickly. Shorter lengths are ineffective.
I have been using Mouse Locator, but I am adopting what I described above. In addition, I use other techniques that people could try out:
1. White-On-Black – because my eyes are very sensitive to light colors. If necessary, I reverse the colors temporarily with the ⌃⌥ ⌘8 short-cut key. It may be necessary to activate this short-cut key in System Preferences > Keyboard > Keyboard Shortcuts > Accessibility > Reverse Colors (if you are on OS X 10.8).
2. A neutral background such as the solid color ‘Solid Aqua Graphite’ This background remains almost the same if you use ‘Reverse Colors’ as mentioned above. See System Preferences > Desktop & Screen Savers > Desktop.
3. Zoom on a large screen (mine is a 24 inch iMac). I need about 5 times magnification to read the text easily without glasses. The pointer is somewhat increased in size independently. Plus, I use ‘When zoomed in, the screen image moves continuously with the pointer’ on a ‘fullscreen’ zoom. All these options are available in System Preferences > Accessibility. Use the Zoom short-cut keys to adjust the screen sizes quickly. They are ⌥⌘= for larger, and ⌥⌘- for smaller, and ⌥⌘8 to toggle on and off. I also set the Zoom maximum and minimum sizes to the same size to provide a default size to decrease or increase from that default size. Do this in System Preferences > Accessibility > Zoom > More Options in OS X 10.8.
4. I also use a ‘screen rim functions’ technique, whereby I have my Dock hidden on the left, many icons on the right side of the menubar at the tope of the screen (there are System Preferences checkboxes to enable them), and Finder > View > View Options > Sort by Name to keep the desktop files tidy on the upper right of the desktop. All this has the effect of reducing clutter on the desktop.