To learn a new passage, find a tempo that is below the upper limit of your control. If you are continuing work on a passage that you have already started, choose the tempo at or just under the tempo that you reached during the previous practice session.
Set a standard for yourself to achieve before allowing yourself to move on to a faster tempo; for example, six perfect repetitions in a row. If a mistake or pause creeps in, slow down, fence it in, fix it, and then go for the six perfect repetitions again. If mistakes are persistent, slow down even further and try again.
After six perfect repetitions, write down the tempo in the margin, increase the tempo two to four beats per minute, and then work towards another six perfect repetitions.