I've mentioned before how I approach clearing my Pokemon inventory. I take in to considering my Pokemon Go goals and transfer everything that do not fit within those goals. This might already be the process that you use now but I know I did not have this way of doing things at first. I always have to remind myself to process it in this manner whenever I have a cleaning session.
Transferring unfulfilling Pokemon lets you enjoy the game more. You are no longer trying to find uses for everything you catch and you can better appreciate the Pokemon you do keep. An extra benefit is gaining more space if you're running low on space.
I always tried to justify keeping certain Pokemon because of the possibility they would be useful if the game's meta shifted. I realized I was using this reason for a majority of Pokemon and it ended up taking space. With this in mind, I went ahead and transfered Pokemon like Jynx, Butterfree and Arbok, not rare but still uncommon Pokemon. I did not particularly care for these Pokemon and it was doubtful they would ever be top tier. But I kept Pokemon like Kingler, Seaking, and Starmie because one of my goals is to have a water Pokemon team.
If one of your goals is to keep a living Pokedex or to min-max your team, then you should follow those goals. Knowing your goals is a reminder to have a clear reason for keeping Pokemon rather than a fear of missing out.
I've taken this idea from Marie Kondo's "The Life-changing Magic of Tidying Up" and slapped on my own name. The Joy Method is a a way to quickly determine whether to transfer a Pokemon. Look at the Pokemon and answer this one simple question:
Does this Pokemon spark Joy?
Do you care enough about this Pokemon to not transfer it? If not then good bye. I'm not saying you should tranfer all the unexciting Pokemon or keep every single favorite Pokemon. It's a fresh perspective on your Pokemon that could be helpful.
I've seen many people talk about how they are preparing for when Niantic adds trading. Trainers have been keeping desirable Pokemon like regionals and Pikachu with hats with the intent to trade later. It's fine to do this but keeping hundreds of them might not be necessary. Players can always catch their own regionals and event Pikachu could always come back. As for me, I do keep some of these exclusive Pokemon but with the intent of trading them to people who want them.
Heres an example of the questions I ask when deciding to transfer a Pokemon. If I go down the questions and 'no' is the answer to the last question, I'll transfer it.
Now, I don't actually ask these questions for every Pokemon but I usually can have them answer 'yes' to one of these questions.
The frequency at which you should free up Pokemon space matters as much as what you clear. If you do it daily after a short Pokemon session, you might mistakenly transfer something for the sake of transferring. If you wait until your inventory is full, you'll spend a few hours checking stats and comparing. There needs to be a happy medium where you can get an idea of what catches you have to work with and save time.
Don't get comfortable if you have a lot of free Pokemon slots. It becomes a time consuming task to clear it and you will eventually fill those slots and need more space.
I've outline my way of clearing my Pokemon and sometimes I don't follow every rule. I still check IVs too often and will have to remind myself to just let it go. I worry that I'm transferring a perfect IV Pidgey and I'm missing out. However I realize a perfect, low CP Pidgey doesn't spark joy for me and I would rather just transfer all of them in exchange for a peace of mind.
Consider these points and see what works for you.
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