If your overall site wide CTR is higher than 5%, then you are doing things right.
If you are below 3% you definitely have some improvements to make.
You want your ads to be related to your site. If they are off-topic the people searching your site probably wont click on them.
Here are the tips:
Create one topic per page.
Mark some of your keywords in bold or italic.
Use your keyword in your page title.