If your track is in a major key, then just lay out your notes in order (e.g. 'c major' C-D-E-F-G-A-B) then the minor scale that will work best to compliment the major root will be the 6th note in your list. So, for example, A minor (6th in the list) will work good with C major.
If your track is in a minor key, then just lay our your notes in order (e.g. 'c minor' C-D-Eb-F-G-Ab-Bb) and the major scale that will work best to compliment your minor key will be the 3rd note in your list. So, for example, Eb major (3rd in the list) will work good with C minor.
If you use this system with your example of A minor, you will see that C major is a good fit for it!
This system works well because, for example, Eb major and C minor use exactly the same notes in their scales, but just have different ordering of the notes, so even though you're mixing major and minor scales, you're not using any additional notes, it's just that the riffs/melodies start from a different note.
This is just a rough guide on what people usually use, the best way though is just to experiment. The most striking music always comes from experimentation and laying down notes that people aren't expecting. Hope this helps!