I worked on my startup part-time for 9 months before taking the plunge and quitting my day job 5 months ago, and "half-assed" is pretty accurate, even if it is a little unpleasant. In terms of the scope of what you can accomplish, you're severely handicapped by moonlighting, much moreso than the difference in available time would suggest. Being able to think nearly continuously about your project lets you try much more ambitious projects, which you wouldn't be able to hold in your head otherwise.

One thing that helped me was to think of my salary as "buying time". If you make $60K but live on $20K, then each hour you work buys you two hours of startup time. If you make $100K but live on $40K, each hour you work buys you an hour and a half. Shows you how important it is to get your living expenses down.

Another thing that helps is to live like your least-successful classmates while working like your most-successful classmates. I have English-major friends who graduated, couldn't get a job, took a string of temp jobs and secretarial positions, spent as much time on unemployment as off, rarely had money to spend, and went back home to live with their parents. So, I went back home to live with my parents, rarely spent any money, and worked at a financial software startup. I pocket the difference between a Starbucks salary and a financial software salary. That's given me about 4 years living expenses to work with, even assuming I were to move out of my parents.