1. Always practice musically!
Be careful! You can never say "it's just a scale" or "just warm-up exercises". Practising musically means varying your touch and articulation, playing with different intensity of the sound. A scale involves crescendo and decrescendo as you play ascending or descending. As well requires staccato and legato, marcato and tenuto, however subtle, are all expressive means that belong in the scale practice regimen of every pianist.
2. Never play too fast!
Evenness is much more important than speed. Speed will come over time, all by itself. Prepare every finger and notes before to play it! The best preparation gives the best attack.
3. Develop an accurate inner pulse!
Regularly practising with a metronome is desirable, whether you play slowly or you need to speed up the tempo when practising in final/required tempo. Musicians need to be equipped both to play strictly in time and to deviate from absolute time in a beautiful but controlled manner.
4. Practice different rhythmic groups.
Practice piano scales first in quarter notes, then eighths, dotted notes (long-short and reverse short-long) then eighth-note triplets, then sixteenths. Even slightly accenting every fifth note can be a challenging change of pace for advanced students. See below some rhythm patterns you can use to practice scales.
You can also apply these principles to arpeggios, making them perfect!
5. No flat fingers!
Play on the tips falling directly from above on the key. One single movement on your curved fingers. The hand won't be flexible and able to play fast and expressively in a flat finger position. Keep your wrist level with your arm.