Adam Callinan has some straight-talking for what he calls the 10 things entrepreneurs need to stop doing.  Our favorite:

You only have 100 percent of your time to split up between your
professional activities. If you do too many different things
simultaneously, you’re just splitting up your 100 percent into pieces
that ultimately resemble slivers of poor performance. Instead, spend 100
percent of your time and focus on becoming excellent at one thing.