How I did it was very simple. I found where my customers were and I lived and breathed on the industry forums they were on. While developing my beta I had a very strong sense of the problems people were facing, how my product solved those problems, and why my product would be superior to competitors. Plus I was able to establish myself as a reputable person within those circles of people on those forums.
So when my beta was ready I was able to get people to pay for the beta by offering them a lifetime discount and the ability to talk with me throughout the development process so that they had some impact in shaping the product. Just from those beta users I was ramen profitable and I was able to bring on an intern for a couple months to help me speed up some of the development.
By the time I publicly launched I had a core group of evangelists (the beta users), a strong reputation on industry forums, and an amazing product that solved big issues my customers faced and I reached 5 figure MRR within a month of launching.