Oftentimes I make a big commit that I later realize should be split into two commits/patches. I think the usual way to split a Git commit is the reset/“add -p”/commit sequence, like this,

# assuming HEAD is the commit to split,
git reset HEAD^ # first undo the commit
git add -p      # then choose bits to separate out
git commit      # commit separated changes
git commit -a   # commit other changes

But lately I've been using another way that involves a “commit –fixup”/revert/“rebase -i” sequence, like this,

# assuming HEAD is the commit to split,
# first, in Your Favorite Editor, delete everything that you want to separate out
git commit -a --fixup HEAD        # then commit the changes you removed as a fixup
git revert -e HEAD                # revert the fixup; this will become the split commit
git rebase -i --autosquash HEAD~3 # combine the original commit and the fixup

One thing I like about this second method is that it lets me pick out the changes to separate out in an editor with full context. I think this is a lot better than git add -p, which involves editing the hunks directly, and sometimes provides too little context to let me determine if a particular hunk should be separated out or not.