Separating portfolio-worthy work from work that actually pays well

The projects that look impressive in a portfolio aren't always the ones that pay well. A routine maintenance contract can fund the month, while an exciting branding assignment consumes three times the estimated hours. I don't want to choose work purely by hourly return, but I also can't pay bills with admiration. How do you balance interesting showcase pieces with boring reliable work without ending up permanently undercharging for the fun stuff?