The ability to edit your own work is incredibly important. For obvious reasons, editors prefer authors who can turn in clean copy. Beyond lightening an editor's workload, there are other reasons that you need to be good at editing--it's your name on the book!
Over the years I've picked up great ideas for editing. Here's the best summary of what you should be doing--
Edit your work in passes.
In other words, don't go over it once and consider it done. Read it several times, for several different reasons. Read it first for pacing and continuity. Read it again for logic. Finally read it to proofread for grammar, spelling, etc.
Let's break that down:
1. Read it for pacing and continuity. Does it drag? Does every scene contribute and move the plot along? If you delete a scene does it matter? (If the answer is, "No," then the scene is going to drag the plot down.) Can you up the tension in the scene? Can you use a chapter break to create a mini-cliffhanger? Are the characters consistent in their behavior?
2. Read it for logic. Are the sequences of the sentences appropriate? Do they build on each other? Do they happen in a logical order? Are there any questions left unanswered? Does your premise and the premise of your characters make sense? Do you maintain an internal logic? And finally, if someone walks into a room, where do they go? What happens to them? Did you maintain a logic to their arrival and departure?
3. Read it for grammatical errors, spelling errors, and consistency of style.
It takes forever to edit a book this way. Unfortunately, a poorly edited book is a book that won't be enjoyed by your reader, so make the time!