Do you know when to say no?

Tough topic.  The thinking is that every opportunity will be beneficial.  Will it?  Really?

Accepting Project A may return a net of $135 per hour.  Accepting Project B may return a net of $225 per hour.  You can do A or you can do B but trying to do both is not realistic.  Too many try to do both and hope nobody notices the problems that underperforming can create.  Sloppy work and shortcuts do nothing to help your reputation and all you have to set you apart from others is that quality reputation.