There's nothing quite like slaving away all day, completing on-call shifts and doing work on the weekends now and then, and being able to save barely any money towards buying a house instead of renting (as renting is a mug's game in the south of the UK).
Patrick McKenzie's advice is good, see:
And I've also read some really good advice here on HN.
My own personal experience: Based on what I believed to be an unmet need/new opportunity, I built a pretty small project. With help from the university I graduated from, I got contacts with the local business community, and now I have more leads (not yet customers) than I have time (I'm currently full-time employed).
Businessmen were really kind and gave me 15 minutes to listen to their needs and pitch my product. We over-estimate the difficulty of securing a meeting (that's not the same as securing a sale though).