In response to @HansOngchua regarding how to best use a metronome for efficient practicing, I came up with this list, which was too long to fit into a Twitter message:

1. Plan your practice session. Organize which sections of which pieces you need to work on. Figure out how much time it is going to work on each section and to play through an entire piece if that’s part of your plan.
2. Practice in chunks. Don’t just set the metronome and plow through your music. Mark off the sections that need attention and deal with them separately – working out the kinks – before you try to play the piece all the way through.
3. Start your metronome at a tempo where you can play the section absolutely flawlessly – everything is in place: technique, rhythm, notes, phrasing, tone, etc. No compromises. Starting anywhere faster and you’re going to be practicing making mistakes.
4. Take the above tempo and lets assume it takes 4 minutes to get through it at an 8th-note tempo of 60 bpm. Up to the target speed, it takes 1 minute at 120 bpm. If you’ve set aside 10 minutes of your practice schedule to work on this passage, then you should set the metronome for ♪=60, ♪=80, ♪=100, ♪=120.
5. Don’t set the metronome for faster than you can play it. If the above scenario is unplayable at ♪=120, try a lower target and compress the in-between metronome markings to fit proportionally.
6. Subdivide. I indicated eighth notes above, but these could just as easily turn into quarter notes, half notes, or whatever. It is not uncommon to start in eighth note subdivisions and wind up later on in quarter note subdivisions.
7. Make sure the metronome is loud enough. Plug it into speakers or headphones if necessary.
8. I almost never use a metronome when playing a piece all the way through. The exceptions to this are when I’m learning notes and want to build technique for a work up to a certain point. But after a while, I break it out into separate sections to work on, so that I can keep certain parts open for rubato, phrasing, and pauses.
9. But the key question @HansOngchua asked was of course how to keep tempo during a performance. Unless the piece is some robotic vivacissimo etude, I think the tempo is likely going to fluctuate a bit based on human interpretation regardless. But in general, I try to keep an internal sense of tempo. When I’m performing a work, I go back to that internal sense of tempo from time to time to get things back on track. I think this has less of a chance of being “off” when the performer is confident and not distracted by nerves.