Practice transcribing music. There are also simple ear training applications you can get for your phone. Relative pitch is a fairly simple concept, so there's a

Practice transcribing music. There are also simple ear training applications you can get for your phone. Relative pitch is a fairly simple concept, so there's a proliferation of apps that teach it, at least to a basic level. It's also common to have a library of songs in your head that start off with each interval. Everything from "Für Elise" for the minor second, to "Somewhere Over the Rainbow" for an octave.