Derek Wyatt shared this in a recent comment, and I thought it deserved it's own tip.
If you're in insert mode, and you hit ctrl-o, Vim will accept one normal mode command and then return you to insert mode. Two examples given showed some typical usage:
<C-o>ma - Mark the current position and then keep typing.
<C-o>gql - Format the current line and keep typing
A final blurb from the comment:
"The next type you do <Esc>(thing)a you might remember that you could have done that with <C-o>".