How do I configure git to ignore some files locally?

From the relevant Git documentation:

Patterns which are specific to a particular repository but which do not need to be shared with other related repositories (e.g., auxiliary files that live inside the repository but are specific to one user's workflow) should go into the $GIT_DIR/info/exclude file.

The .git/info/exclude file has the same format as any .gitignore file. Another option is to set core.excludesFile to the name of a file containing global patterns.

Note, if you already have unstaged changes you must run the following after editing your ignore-patterns:

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

Note on $GIT_DIR: This is a notation used all over the git manual simply to indicate the path to the git repository. If the environment variable is set, then it will override the location of whichever repo you're in, which probably isn't what you want.

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Edit: Another way is to use:

git update-index --skip-worktree <file-list>

Reverse it by:

git update-index --no-skip-worktree <file-list>

Update: Consider using git update-index --skip-worktree [<file>...] instead, thanks @danShumway! See Borealid's explanation on the difference of the two options.

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Old answer:

If you need to ignore local changes to tracked files (we have that with local modifications to config files), use git update-index --assume-unchanged [<file>...].

Add the following lines to the [alias] section of your .gitconfig file

ignore = update-index --assume-unchanged
unignore = update-index --no-assume-unchanged
ignored = !git ls-files -v | grep "^[[:lower:]]"

Now you can use git ignore my_file to ignore changes to the local file, and git unignore my_file to stop ignoring the changes. git ignored lists the ignored files.

This answer was gleaned from http://gitready.com/intermediate/2009/02/18/temporarily-ignoring-files.html.