Mine's kind of sticky as well. I replaced the ignition tumbler assy. (I did a detailed write-up on it if you search) and it didn't help much. There's something in the parts that engage/disengage the locking pin that's failing. I'm almost to the point where I'm going to start tearing into it because its starting to annoy me now.

FWIW, I find its easier to turn the key if you hold the steering wheel slightly to the left while doing it.