- include keywords that are related to the websites your customers visit
- use agressive targeting to identify new keywords
- only include exact match keywords
- only use Keyword Planner to identify new keywords
Explanation: Choosing keywords for a Display Network campaign is about connecting your ads with people most likely to be interested in your business. Based on what you know about your customers, pick keywords that would you imagine would appear on websites your customers would visit.
Read more here: https://support.google.com/adwords/answer/2453986?hl=en