Quick tricks to come up with new melodies

Here are a few tricks to come up with new melodies when you are stuck:

Stay on the chord notes if you want to create "background melodies".
Work with your whole set of scale notes to create lead melodies (you don't need to use all possible notes, but you also don't have to stick just to the chord notes).
Try this: Create a rhythm first, then change the notes until it sounds good. In Sundog this is very easy: Create or load a pattern that only consists of base notes ("0"), then draw in a "melody contour" in the Pattern Trigger editor.
Slow, long notes that don't cover a wide range of notes -> calm melodies, epic melodies.
Quick notes that change often and maybe use two or more octaves -> happy melodies, energetic melodies.
Do you need more tension in your song? Write melodies where you also use notes that are just one semitone apart. Example: The little four-note-hook "C -> C# -> C -> C#" sounds quite scary. All the classic Western scales contain some notes that are only one semitone away from each other.
A common technique is called "call and response": Create a little melody, then "answer" with another little melody. Example: Create a small arpeggio that is going up... wait a bit.. answer with a small arpeggio that is going down.
A good way to create memorable melodies that aren't too annoying: Create a small melody. Double the number of steps, copy the melody to the second part. Now do a little twist at the end of the second part.
In most cases it's a good idea to use a simple base rhythm + melody and then throw in some more complex parts. This way the melody can be memorable while not being overly simplistic.