gitignoretip.txt

If you are trying to ignore changes to a file that's already tracked in the repository 
(e.g. a dev.properties file that you would need to change for your local environment but you would never want to check in these changes) than what you want to do is:

git update-index --assume-unchanged <file>

If you wanna start tracking changes again

git update-index --no-assume-unchanged <file>


Update: Since people have been asking, here's a convenient (and updated since commented on below) alias for seeing which files are currently "ignored" (--assume-unchanged) in your local workspace

$ git config --global alias.ignored = !git ls-files -v | grep "^[[:lower:]]"