QA here. Any time you find a bug, write a test for it. One of the simplest strategies.

Also there are so many static analysis tools. FXCop etc. Been at so many companies where there are thousands of warnings in the code, many of which were genuine bugs. At one, we fixed 16,000 warnings, and managed to close off a few dozen sev 1 crashes that people had been experiencing for ages.

Start by letting your customers report bugs. Give them an easy way to contact you. Cost will be free.

Ensure that you fix the bugs and write some automated test for this case if possible. Again free.

Pay someone to do a little QA of your product. I'm sure it'll hurt to give away money that you could pocket yourself... but it's the price of getting work done.