It's easy to become a one hit wonder, staying relevant is where the work comes in.

Be Unique

This is where most musicians fail. They keep trying to sound how other famous people sound because they want to be famous too. Why try to be like someone else? The public will want that someone else, not a replica. Being unique is where it all starts.

Don't be complicated

Just because you understand Mozart, doesn't mean you are Mozart. Either way, radio won't play your song if it has a million different melodies and chords 9th 11th Sharpened 5th whatever it may be.

Be Relatable

You will never ever relate to absolutely everyone, get that out of your head. The subject that will relate to most people is love which is why 90% of songs out there are about love. So yes you can write about love, about money, but you may end up sounding like everyone else. Just write what you want to write, tell a story and it will probably relate to a lot of people out there.

Keep it short and catchy

3:30 seems to be the perfect hit song length. Every hit song has to have a hook, something people will remember it by. This is usually in the chorus, and hinted in the versus.

Marketing

The perfect way to market a song changes all the time. I can't guarantee what I tell will work, but it won't hurt you to try it out.

1. DON'T SPAM PEOPLE. If you're constantly sending out album art people which slowly but surely see it, get over it and scroll past. It becomes redundant. 2 or 3 posts on social media, that's all you need.
2. Start a hashtag. Get 10 to 50 of your friends to create videos that relate to the story of your song, and upload using that hashtag.
3. On the day of the release, get all those friends to constantly tweet or upload with that hashtag as well as a link to that song. MAKE IT TREND.

I don't have a PhD for song writing or writing hits but this way works for me so try it out!