not much to go on there so can only give you a basics check list.
Make your adgroups are tightly targeted, i.e keywords are all for same product or service, or even a specific aspect of a product or service, and use matching options, i.e not just "credit card"adgroup with everything thrown in but maybe many credit card orientated adgroup such as one for balance transfer credit cards with related searches in it like :
"cheap balance transfer credit card"
"cheapest balance transfer credit card"
"cheaper balance transfer credit card"
etc.
Make sure your adcopy is highly targeted, use the default text of "balance transfer credit card" but maybe use dynamic insertion too {KeyWord:Balance Transfer Credit Card} so that the actual phrase searched will usually appear in the adcopy, but if it's too long then the default text after the ":" will appear, make sure you sell the sizzle in your adcopy, tell them what you offer and what it will do for them, but also bear in mind asking questions in adcopy can be as good as telling them facts too if you ask the right questions, Use two of more variations of your advert, leave a few days then change the worse performing one again.
Make sure your choose the right landing page on the merchants site, if more than one is available always send traffic to the most relevant landing page.
Add as many negative keywords as you can find, the more you add, the less irrelevant searches you appear for, which means you Click Through Ratio will be far higher, the higher the CTR the better off your advert and keywords are treated as google scores them as more relevant.
Change your bid to move your ad up and down, first place is rarely best for high comparison items from an ROI point of view so experiment. find the sweetspot .. 2-4 maybe best for volume and conversion, whilst position 6-8 may yeild the best ROI for your adgroup, is all about testing and finding the right combination of keywords, negatives, adcopy, ad position, merchant and landing page and cost v roi that makes you happy.
Another thing to consider is that the merchant you have selected may not be the best one available in the marketplace for your traffic and whilst commission payout is important it's the actual earnings per click that counts, so are there other merchants to consider, even if they pay less ?, you could still end up earning more if they convert better, find one and alternate traffic between a them or you may fare better by doing a site of your own, listing the same product or service from a few merchant sites and sending traffic there, for the user to decide which one they like.
AdWords is like 3D game of chess
and you won't win every game but over time you'll get better with practice but you need basic strategy to start with so reading everything you can about adwords and ppc in general is a good start.
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