1. Conjunct motion.  A nice melody usually moves mostly by step (adjacent or nearly adjacent keys on the piano, for instance), and more rarely by leap.

2. Avoid stuttering.  There often is a more elegant solution than repeating the same note or wiggling back and forth among the same two or three notes.

3. Phrase shape.  Many folks prefer phrases that have a definite beginning, climax, and end.  The climax is usually the highest note of the phrase, but sometimes the lowest note.  Climaxes are usually more convincing if, within a phrase, they're unique: the melody reaches the extreme only once.  Phrases are often grouped together into larger units with their own beginning, climax, and end, and again, the climax is a unique one of the phrases.

4. If you've already got a series of chords, an easy way to sketch the backbone of a melody would be to choose notes from the chords. 

If you're making classical counterpoint, your series of chords will be a bit more specific in voicing, and you'll want to make sure your melody lines don't introduce things that classical counterpoint shuns, like parallel fifths, ugly doubling, etc..

5. The backbone can generally be decorated in the following 3 ways:

a) Arpeggiation.  A chord tone can he replaced with a series of notes within  the chord.
b) Ornaments.  A melodic tone can be replaced with a more-or-less conjunct wiggle containing and emphasizing the melodic tone, e.g. beginning and ending on it.
c) Passing motion.  Two melodic tones separated by a leap can be replaced by a scale beginning on the first note and ending on the second.

These three expansion rules can be applied recursively to the notes of an already ornamented melody.  Classical style would require, once more, that the added notes never introduce anything that classical counterpoint shuns.

Classical musicians further subdivide the world of ornaments into a number of different categories.  Some theoreticians (notably Koch and Schenker) feel that a listener can and should be able to sense the hierarchy of ornaments recursively applied, and that creative use of these structures can in itself be expressive.

Non-classical musicians usually use some but not all of these ideas.  For instance, what classical musicians call "stuttering", chanting repeatedly on one note, is an important core of reggae, an opportunity to highlight vocal rhythm.