For aural skills:
- First, ALWAYS engage in active listening---sing under your breath along with everything you hear, and attempt to place solfege syllables to the music. Especially this applies to basslines; harmonic hearing begins with singing the root of the chord in pop music, and this will be the best training to open your ears and mind to harmonic thinking.
- Second, practice singing the Monte at a piano, and don't be satisfied unless each note is slotted correctly. Real music modulates, and being able to do the modulation yourself, in your head, will make you supremely powerful.
- Third, do transcriptions and arrangements of music to sing/play/perform with friends! This is the essence of musicianship.
For music theory:
- First, make a REAL effort to get into classical music! Music is a language, and music theory describes the grammar. But most important is to learn to speak it fluently, and just like with languages, immersion is the key. Make a playlist of Bach inventions, Haydn symphonies, Mozart string quartets, Beethoven sonatas, Schubert lieder, or Chopin Nocturnes! The beauty of this music is timeless, and the melodic and harmonic games you'll find are sublime. This is the single most important thing you can do as a musician.
- Second, learn to play some piano---the visual map of the keyboard will help worlds in thinking intuitively about music theory. Plus it's good for parties.
- Third, write music. Grab a guitar and sing, write a trumpet impromptu, or even a classical-form minuet. And remember that your first really good composition will probably be at least your tenth, so there's no pressure to be instantly amazing.