Set a timer! Decide what you’re going to work on and do just that and only that for the time set on your timer.
To illustrate the idea, the following are made up scenarios in the context of classical guitar but can obviously apply to any instrument, vocals, or activity for that matter.
That difficult phrase from bar 15 to 18 in the Bach Partita you’re learning. Set the timer for 5, 10 or 15 minutes, loop it over and over only stopping to pause to repeat the phrase. Don’t do anything else but THAT for that allotted time until the timer goes off.
Another scenario, say you’re just starting a piece and have yet to work through right hand or left fingering decisions. Put the timer on for 60 minutes and only do that for that time.
It works great for technique practice too. Say you’re going to warm up with rasqueados or right hand finger alternations: put a stopwatch on and do ima, ami, im, mi, ma, am, ia, ai combos for 60 seconds each equaling a quick no nonsense 8 minute righthand warm up.
In preparing for a gig/concert sometimes it’s essential to practice how it’s going to be on stage just playing through the material at tempo. This will mean very little time between pieces and not getting up if seated or sitting if standing for perhaps an hour or longer at one time for a set. How often do we practice like this? Not often, usually there’s coffee, FB checks, texts, our dog wanting a pet, etc. Here again the timer is key, don’t do any of these things during the 60 minutes or whatever time it is set to until that buzzer goes off. You are practicing for battlefield conditions and the timer is your drill Sargent.
Be creative with a timer and use it to keep focused and to spend enough time on the parts of practice that are paramount for repetition and time spent. Without the stern objectiveness of a timer it can be too easy to deceive yourself into thinking you’ve spent more time than you actually have.
Last and most important, don’t ALWAYS use a timer. This is for the same reason you wouldn’t always use a metronome, both are tools with a means to an end.