Submitted my portfolio - but I was already working for DC Comics at the time so it made it a lot easier to "prove" I was a professional. My first Marvel work was in Excalibur 83, summer of 1994! I started working full time in comics when I was still in college.
I drew lots of pages and took my portfolio to 2 shows a year where I tried to get as much feedback as possible from pros. I would then go back and draw another set of samples and not to make the same mistakes. I kept this up for a few years until the pros would have to get so nit picky with me to find something wrong, and was told to find work!
Also, while in college I took tons of drawing, painting, design and art history classes for fun and by my 4th year in school I was working full time in comics.