Getting pin sharp focus

Hi everyone,
I recently did some portrait work, but was disappointed that the focus wasnt as good as it should be. I tried everything i could think of, but couldnt improve the results.
Can anyone advise on how i can improve my focussing?
Thanks as always
John

Post one of your images so we can see what you're talking about.

Seeing what you mean and knowing what you're using would help a lot. Also knowing if you're hand held or on a tripod and what focus mode you're using. Handheld you need to make sure you don't move after getting the focus. On a tripod if the VR is turned on it's going to actually induce movement which will impact it. The more we know the more we can help.