Spell-check and proofread to correct grammar errors. If your text has
too many of these, it can cost you in terms of assumptions the reviewers
may make regarding your general attention to detail (and thus whether they
trust that you conducted your analyses carefully). If your English
is not fluent, it's worth getting help with it. Unfortunately, for
"blind reviewing" (conferences that require anonymized submissions),
deficiencies in English can prejudice the reviewers towards expecting
inferior quality in the work. Conversely, engaging writing can really
help in subtly suggesting to the reviewer that you know what you're doing
and they should trust you on unclear fine points.
To automate some of this,
you might want to check out
this tool by UCB grad student Devdatta Akhawe,
these scripts, and,
if you're an emacs user, flyspell or
writegood modes.