Me i just move up from the key of the main melody and play in a scale.
In a classic sense, start with:
a) Play in the same key as the rest of the music.
b) Split your lead melody into phrases.
c) Start most but not all phrases on the root note of the key.
d) End most but not all phrases on the root note of the key.
e) Leave spaces without notes to let the melody breath.
f) If you can't hum it, it's too busy.
g) Ignore these points of you feel like it.