Here's a simple rule for productivity: As much as possible, focus on doing things that energize you and avoid doing things that drain you. If it excites you, say “yes.” If it doesn't, say “no.”
If you focus on doing the things energize you, then two things will happen:
On the other hand, if you focus on doing things that drain your energy, then you'll likely be more miserable and, even more, will do those things half-assed, which will only demotivate you.
This is something that I myself apply to any advice given to me. If someone tells me that I should do x, y, z to achieve some result, I pay attention to how I feel about it. If it excites me to try it out, then I will do it. If it doesn't, then I don't.
Of course, you'll have to break this rule from time to time. Sometimes you will have to do things that you don't really want to do. But as much as possible, you want to maximize your time so that you are mostly doing the things that energize you and avoiding the things that don't.
This idea is simple and doesn't need much explaining. Yet it's probably the most important rule you can practice in your own life because it'll help you not only manage your time, but make life so much more enjoyable.