I've been at it for like a year and 9 months now, though not particularly trying to make it a main aspect of my life per se, and I have like 100 followers. Which I have no idea if that's good or bad for 1 of probably a million random music tumblrs
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I don't really get much in the way of feedback so I'm not 100% sure what truly works, but I think the main things are:
1) Have a voice. Don't be afraid to make your subjectivity and personal feelings known. I purposely only post about stuff I like since I find no point to negative reviews, but I still take random jabs here and there. Just try not to make it too dry. add personality.
2) Always add tags so people can come across your blog. Add like 15 at least, too.
3) Post timely stuff so people are actually searching said tags. Obviously not every artist you want to talk about will have something going on, but don't make a post about an artist 2 weeks before an album drops unless you wanna post about them twice. More people will theoretically be searching the web for info on that person when the news is fresh.
4) Understand that very, very few people will ever care what you have to say. Write because you enjoy writing, not to gauge your self-worth by web traffic.