Uhhh, where to start? A good do: Finish your story completely before starting posting (you'll invariably have to go back to ealier parts and change them as new good ideas come along). Another good do: Get someone else to preview before posting (another pair of eyes will always catch things that have become - literally - invisible to you as you edit and re-edit countless times).
Apart from that, there is an entire library of "how to write" books lol. I'd say the Turkey City Lexicon is a nicely condensed list for starter writers, and is readily found with a Google search. SFWA also has regular interesting columns for SF writing.
Hope this helps!
K