It is not something than one can improve as such…. as we are all born with a fixed number of follicles which gradually and steadily reduce in number over time.. Controllable factors are smoking and excessive drinking!
If you can, avoid repeated ovarian surgery. As multiple cystectomies with excessive diathermy use is related to decreased reserve. However some conditions requiring cystectomy may also contribute to reduced ovarian reserve in the first place eg chronic endometriosis.