Sometimes we need some extra help answering a pesky grammar, punctuation or citation question. Here are my three favorite websites that I recommend regularly to composition writing students:
Purdue University Online Writing Lab
If you are an enrolled student, see if your school offers tutoring services, in-person or online. This is a great resource for additional, usually free, one-on-one customized help.
If you are working on something for publication, you might consider hiring an editor. I highly recommend my friend and colleague Amy Bucklin of Clear Sky Writing.