How to increase Test Coverage?

I have often been asked by my clients to increase application test coverage through testcases and reduce customer defects and i wish i have a magical wand to help them.

In my perspective, increasing test coverage depends on various factors. How knowledgable testers are on the application, how effective they are when it comes to understanding of new requirements and creating testcases for that, do they step into the customer's shoes during test design are some of the key aspects which help in increasing test coverage.

Testcase effectiveness is a metric that talks about how effective a testcase is in finding defects and any testcase that is executed repeatedly over a period of time without identifying defects should be reviewed for its effectiveness. And, to design an effective testcase, the tester's knowledge and his approach of the application from the customer's viewpoint definitely matters.

I have seen products that have more than 10000 testcases for the application but repeatedly after every release customers file many defects which should have been ideally caught by QE(Quality Engineering/Testing team). And needless to mention, all these customer defects are being created as testcases after every release and are added to the test execution bucket for the next release to ensure that the same defects are not repeated and for increased test coverage. This eventually results in a lot of testcases being created but somehow the end goal of "coverage " never gets fulfilled.

So, the question is, how do we bring 80/20 rule here? 20 percent of the testcases should find 80 percent of the defects in the application. Because unless we do that, execution will be a laborious task with no goal and at the end of the day testers will still say "I have not tested this part of the application well and hence could be a risk". So, how can testers provide customers with a quality product by testing as much as possible within the given execution timeframe?

p.s: I am definitely not looking at 100% test coverage.