Something a lot of people get wrong about PPC...

[MaryWhite20]

PPC is all about relevance. Users are searching for specific products, services, and information at any given time. Advertisers have the ability to show a targeted ad at the exact moment this search is occurring. For example, if a user searches for “housekeeper service,” an advertiser can show an ad speaking to that exact search.

If you're not relevant, you shouldn't be wasting your money advertising via PPC. That is, in my opinion.

[riyajindal]

Targeting is the main thing.
Targeting your ads to your specific ad group is vital.
If you have not targeted your ads properly, your chances of boosting your conversion rate from them will be very low.

[Arpita]

Bidding Broad
Avoiding Geo-targeting
Losing Relevance on Landing Page
Getting Rid of Fraud Networks
Using Telephone Numbers
Not Bidding for Your Name
Lack of Affiliate Control
Not Separating Content Match from Search Results
Lack of “Exact Tracking”