I often practice something I want done excactly as I hear it. I often use a day or two, especailly on new instruments or an instrument Im a bit rusty on.

Key is to know how it should sound, and then practice it in small chuncks untill you'll get some success.

Break everything down in small parts, and move on once you've gotten those right.

Just spend 15-30min at a time, just so you get from impossible to hard, but do it several times a day or week.

Stop once you got it (and you WILL!), reward yourself with a musical treat (listen to your favourite song, or even better .. play it) and move on to the next thing.

I practice very goal oriented for my demos, and move on once I got the part recorded.