Play them as fast as possible.
See here
http://www.youtube.com/watch?v=BHQYmnA-jbk
This is what you should aim for. The faster you can play, the better musician you are. It's that simple. Once you know a scale, rinse it as fast as possible then move on to the next one. Try and play as many different scales as you can as fast as you can, and then people will know not only how fast you can play them, but also that you know more than one. When you're playing a scale, don't mince around going 'up, down, up, down, lah lah' - Go to the top, or go to the bottom - just get on with it!
Gurning, screaming and sweaty palms will help.
If you want to play a scale REALLY fast, then you might want to leave out the boring notes along the way. This is called an arpeggio. Arpeggios are even more impressive than scales, but again, remember to play them as fast as possible.
If you start from a different place than 'the start' in a scale, you have a mode. Mode means your in a certain mode - like being in "super pursuit mode" if you're knight rider's car. Modes in music are useful for showing that you understand emotions. There are seven emotions, and hence there are seven modes.
1. Lydian - airy fairy elf lovely
2. Ionian - everything makes sense today
3. Mixolydian - feeling kindy cheeky, firm and fruity
4. Dorian - duh, am I lethargic or just cool?
5. Aeolian - the women are crying
6. Phrygian - orcs and goblins
7. Locrian - will-o-the-wisp, at one with the amoral nature spirits
There are other scales which are even more complicated and these are for 'complex emotions'. A complex emotion is like a combination of the ones above - for example Melodic Minor 'everything makes sense today but the women are still crying' or Lydian flat 7 'my elf is cheeky'. This is high level stuff so make sure you master the basics before you move on. Using complex emotions gives your playing even more depth.
By rinsing a good variety of scales, you can be both a good musician and a true artist, painting vivid pictures with emotionality. Always check you are allowed to play a scale by consulting this diagram
http://www.apassion4jazz.net/jazz-chords-scales.html. There are no wrong notes, but at the same time, some notes are wronger than others. You might hear things like - "Sometimes the wrong note is the right note and sometimes the right note is the wrong note. Ah...." This is called ambiguity.
Below is the wrongness continuum. It is a sliding scale from obviousness to wrongness.
Obvious - - -> - - - Clever - - - -> - - - Wrong
Aim for the middle. This means a scale that comes lower down the list on the chart.
Example for scale choice over a Imaj7 chord (see chart)
Ionian (obvious, duh) -> - Lydian (clever, vg star) - > - Harmonic Minor (wrong, oh dear)
Hope this is helpful.