One of the best self-editing tips I can give is to read your manuscript out loud. It feels silly, I know, but the benefit outweighs the awkwardness.
The human mind compensates for errors. When reading, mistakes are missed because the brain anticipates patterns and corrects inconsistencies automatically. Reading out loud forces the reader to slow the rate, which helps identify errors.
Read the words inside the triangle below silently.
Did you spot the error? Your chance of spotting the duplicate "the" is higher if you read it out loud. This kind of mistake happens all the time in manuscripts, primarily because of cutting and pasting. It is the type of error for which the mind compensates.
Check this out:
Aoccdrnig to a rseearch sutdy at Cmabrigde Uinervtisy, it dseno't mtaetr in waht oerdr the ltteres in a wrod are, the olny improamtnt tihng is taht the frsit and lsat ltteer be in the rghit pclae.
Could you read that? I had no trouble. Amazing what our mind can do, huh?
For me, the biggest benefit to reading my manuscript out loud is spotting unnatural or rough sentences. The rhythm and inflection in my head is different than that of the spoken word. Things that seem natural, particularly in dialogue, often are stilted when read aloud.
I also recommend having someone else read the manuscript to you. Because the characters in one of my novels are teens, I had teens read it to me. Talk about a wake-up call to bad dialogue! Mercy.
I like this editorial tool so much, I read almost everything I write out loud now--manuscripts, letters, emails, blog posts... It not only helps me spot errors, it annoys my family! Who could ask for more? *wink*