One of the best pieces of advice I received this year came from Derek Sivers. Use this rule if you're often over-committed or too scattered:

If you’re not saying “HELL YEAH!” about something, say “no”.

When deciding whether to do something, if you feel anything less than “Wow! That would be amazing! Absolutely! Hell yeah!” — then say “no.”

When you say no to most things, you leave room in your life to really throw yourself completely into that rare thing that makes you say “HELL YEAH!”

Every event you get invited to. Every request to start a new project. If you’re not saying “HELL YEAH!” about it, say “no.”

We’re all busy. We’ve all taken on too much. Saying yes to less is the way out.