Next, you want to sit down with a blank sheet of paper and think of every possible way of searching for the product/service you're trying to promote that is unlikely to result in a sale.
In other words, you're trying to sort the "informational" searches from the "transactional" ones, and the transactional ones you can't convert from the ones you can.
An example (contrived, admittedly): you're trying to advertise a site that sells NEW Mazda MX-5 cars.
Searches containing any of the following are unlikely to convert well:
- used
- second hand
- part exchange
- parts
- aftermarket parts
- value
- accessories
- tuning
- fuel consumption
- restoration
- specific parts references e.g. "tyres" or "hub caps"
Go on adding to the exclusions list until you've narrowed a flood of mostly useless searches down to a dribble of valuable ones.
Remember, even if people don't click (i.e. no direct cost to you) if your ads show up for too many irrelevant searches, that reduces your overall CTR and affects your quality score and hence how much you pay per click.
The way to get (relatively) cheap clicks is to show very highly targetted ads to the narrowest possible audience, already pre-culled for useless traffic. That way, you maximize your CTR and productive clicks, and minimise your per-click cost.