* Play more.
* Play maps just right above your usual star rating, for example: if you play 4 star with over 95% acc, play 4.2-4.3 stars and add in some 4.5 if you can get around the 90% mark.
* Don't retry songs, play them until the end.
Also, don't worry too much about ranks/pp. It's better to focus on getting better while having fun, and that is how you actually gain pp in the end. You'll just have to get better if you want to get more pp, and when you're good enough you'll spike up the ranks quickly and get stuck again.
This is what I do and have since the very beginning of my Osu! journey and I thank the people in the forums and the pinned threads in this section for that
Tip:
If you want to squish out a little bit more pp from your current top scores, get 98% or higher on them and if you manage to get 99% then that's nice too.
Play maps you haven't played around your top scores star rating and get 98% or higher.