What do you want to do?
Before I sit down at my desk to do anything, I ask myself "what do I want to do?"
It can be to work on a project, draft an outline, develop an idea I've had for a client, make a call, do some housekeeping, or something else entirely.
I'm much less likely to get distracted by something shiny, a social media rabbit hole, or an attention-seeking but low-value email, if I decide what I'm going to work on before I sit down at my desk to start working on it.
If I realise that I've drifted down a track I wasn't intending to follow and it's not going to take me anywhere useful, I'll get up and walk away. Then I'll wait until I've got a clear answer to "what do I want to work on next?" before I sit down to start again.
Your time, focus, and attention are very limited resources. They're too valuable to let them drift away unintentionally.
P.S. You can use the same idea when you're ready to take a break. Decide what you're going to do next before you leave. I'm trying to increase the odds that I'll stay focused on what I want to be working on.
P.P.S. I'm not saying that you shouldn't use social media, or look into new shiny toys, or jump into a rabbit hole to see where it goes. (I'm fond of playing blitz chess.) You can't and shouldn't be working all the time. Just be intentional about what you're doing and when you're doing it.