It all comes down to monitoring - knowing your numbers, keeping what works, scrapping what doesn't. Check your stats at least once a day to start.
Use negative keywords to keep unrelated phrases which include your keywords from inflating your ad spend.
Optimizing your landing page is also important. If you have a high click-through and conversion rate, obviously you can afford to spend more for your ads and you will probably benefit from it. Conversion rate is largely determined by the merchant's site (their landing page, quality/relevance of their product or service, checkout or lead submission process, etc.) but your page can influence it more than most new people realize. It's all in what you say and how you say it

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