The over all concept I present to all my students is simply this:
that Harmony and Melody are "Temporal" variations of sets of notes.

Think of a set of notes ( any scale / mode...) in this fashion:

TIME FLOW ------>

H


A


R


M

M    E    L    O    D    Y

O


N


Y


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  BASIC PREMISE # 1:  MELODY = NOTES OVER TIME (ie; PLAYED ONE AFTER ANOTHER).

 All 12 notes are available for MELODIC use, including bends!
Listen to the solos of any great (even 'good') player and you will often
hear all 12 notes being used.

 You may notice that some lines or riffs go "all over the place" in a
harmonic sense - but they start and end in the "right" places.

 Often the rhythmic feel, is more important than the actual set of notes!

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  BASIC PREMISE # 2:  HARMONY = NOTES AT ONE TIME (ie; PLAYED ALL TOGETHER).

 There are 12 notes available (not counting "Micro-tones" or "Bent notes" etc.)
when playing chords (HARMONY). One should try to avoid using 3 adjacent semi-tones
(example; C,C#,and D) as these CLUSTERS tend to sound more like scary-movie stuff
than music. Even if you seperate the notes into different octaves, they still
sound rather awful!
 Now, here is the real consideration- what combinations of notes can be created
based on never having the 3-adjacent-note-cluster?
Well, after a little experimenting, you can reduce those "harmonically viable scales"
to the following few...

The largest scale one can create that allows for "Harmony"
is the 8-Note Diminished scale:

                   1     2     b3     4     b5     b6   bb7     7     8(1)

(Scales larger than 8 notes will have a cluster in them somewhere - do the math!)

Then we have the "BIG FOUR" 7-Note Diatonic Scales:

MAJOR              1     2      3     4      5      6     7     8(1)

MELODIC MINOR                  b3

HARMONIC MAJOR                                     b6

HARMONIC MINOR                 b3                  b6

The 6-Note  Whole-Tone Scale: 


                   1     2      3     #4     #5     b7      8(1)

  Remember this concept; The only harmonically viable scale with more than
  7 notes is the 8-note diminished scale. The only one with less than
  7 notes is the 6-note whole-tone scale. All "other" scales are either
  contained within the above mentioned scales(as MODES) or have the dreaded
  (harmonically non-viable!) 3-note cluster!