One really common mistake is not grouping your keywords into their correct ad groups. An ad group should be a concise collection of keywords matched up with one or two ads that is really focused on just those keywords.
So you’re not going to want red shoes and yellow shoes and blue shoes all in the same ad group, because what you are going to say? “Buy our red shoes and our yellow shoes and our blue shoes”? No. That’s a stupid ad.
What you’re going to want to do is have one ad group for red shoes with an ad that says “Buy our red shoes.” Have another ad group for blue shoes with an ad that says “Buy our blue shoes.” And so on.
When you are arranging your ad groups, try to focus as narrowly as you can. Write the ads that are specific to those narrowly focused keywords.