Research your world meticulously, ensuring that the science written into your book matches with what is currently understood about our universe. Books that focus on the details of the science are often called "hard" science fiction while titles focusing on the characters and situation are called, "soft."
Read the foremost names in the genre in order to determine different approaches to similar subject matter. Consider which of these writers most closely approximates the world view you share and try on a similar approach. Echoing great writers is often a step to developing a voice of your own.
Write your book for a particular kind of science fiction reader in mind. Trying to write a book that will please everyone will never succeed.
Limit your reading of other science fiction books while you are working on your own. Though you want to be informed of the traditions of genre, you don't want to be smothered with competing influences at the actual time of composition.
Set aside time to read your work with a forgiving eye. It is all to easy to become merely critical of a book that is designed to entertain. Doing this will enable you to push on even when the book seems beyond saving.
Remember to be playful and allow your characters to take on a life of their own in the situations and difficulties you expose them to. Trying to force an authorial agenda on a book results in an overlong political tract rather than a satisfying novel.
Join a number of listservs dedicated to the science fiction genre. The Internet is packed with die hard science fiction enthusiasts and many of them will be able to recommend good publishers, both small and large.
Check your favorite science fiction novels to see what houses published them. Genre based publishing outfits are usually very specific in their list needs.
Remain persistent and keep querying appropriate houses. Remember that e-publishers are particularly receptive to work from new writers.