Apply negative keywords to avoid matching to search queries that include "customer service", "contact number", etc. as we want to avoid driving cost for keywords that will not result in a conversion.
Look for common themes of under-performing search terms that you can add as phrase match negatives instead of exact match negatives.
Look for search terms that should match to one of your other campaigns (e.g. If you are running two campaigns, one for running shoes and another for soccer shoes, and you see running shoe related terms matching to soccer shoe campaign keywords, you will want to negate running shoe keywords from your soccer shoes campaign).
When performing negative keyword sweeps, always make sure that you're looking at the search terms report instead of the keyword report to ensure you're creating negatives based on actual search queries and not just keywords that those queries matched to.