I use the following presentation of landing page best practices to dissuade clients from doing the long list of things they are sure will improve their landing pages. You can find a more detailed explanation of many of these best practices in Steve Krug’s book, Don’t Make Me Think. I recommend you read it. You can also gain a lot of valuable insight into effective landing pages from Marketing Sherpa. Most is based on tons of experience. The mistakes we have made can help you avoid making them all over again.
Please add ideas that work for you. I may well have missed something.