It's the little things that I shudder over. How on earth, after all these years, can I mistake cord for chord? I know the difference, but in the heat of writing, I seem to write the wrong one. Too many times. The same is true for reigns and reins. I found a great little site that is old but still really powerful when it comes to making sure I'm using the correct homonym and spelling it right.. http://www.cooper.com/alan/homonym_list.html
Another fun landmine that drives me nearly mad is the use of subjective verbs like was and were. By the way, Grammar Girl (see link below) really knows her stuff. I will take a moment here to say - don't use was and were if you don't need to. Action verbs like stretched, heaped, slid, lopped, burned... all kinds of fun things. One trick I learned at an SCBWI conference: use odd verbs. If you're writing about a person driving a truck, write: The engine's hum shivered through me. My bike flew as I peddled faster and faster. http://grammar.quickanddirtytips.com/subjunctive-verbs-was-i-were.aspx
Oh - another thing. I strive to remember to write the right (little homonym there for you) the verb agreeing with the subject. That's a fun one. If I'm writing about one person, I've got to make sure the verb agrees. Something like this. She does that well. They do that well. Sometimes it's easy. He (or they) scurried over the half-rotted logs. Same verb for both.
Life is focusing.
PS - I get stuff wrong all the time. If you see something here, please let me know. Every learned thing is important to good writing.