I've tried the "just do it" approach in the past and now believe its nonsense. Be calculating and methodical, not impetuous.

Do's/Dont's: Don't work on your stuff on employer time. Make sure you have energy left after work for your project (sometimes that means not being the star performer at the day job). I work my day job from 8-5, and my startup from 6-11 EVERY WEEKDAY. Weekends its ALL DAY developement. Have a product roadmap, release dates, and work your tail off to meet them. Do your architecture design work during the week at lunchtime (off site). Cut anything from your time that isnt helping you meet your goal (ie I dont even drink beer anymore because it hinders coding).

Read this for inspiration (its short): http://loiclemeur.com/english/2007/12/the-idea-does-n.html

Finally, only quit your day job when your startup is making your salary.