Source bids
Raise or lower bids for individual sources based on conversion performance, costs and sufficient sample size. A cheaper source is not necessarily better if it does not convert.
Frequency caps
Limit impressions per user per period, including source-level caps where available. Review caps before launch: very restrictive settings can reduce delivery.
Review changes
Check reports after each change and allow enough data to accumulate. Use blocklists for unsuitable placements rather than repeatedly lowering bids on traffic you do not want.