Maybe worth a read:
Acoustic Guitar: How Guitar Strings Age—and How to Know When to Change Them

I can in fact recommend this quoted action (done on the 4 wound / lower strings, after loosening each one then re-tighten to pitch):
"In a more low-tech approach, one player I know used to aggressively pull and snap each string against the fretboard a few times in a row to dislodge some of the built-up gunk. Though I was initially skeptical, I admit that I heard an improvement in the liveliness of his wound strings after this treatment."

I get maybe 4-5 refreshes of the tone via this method over a similar number of months before replacing a set of strings, with considerable savings on the wallet (doing that for 5 instruments...) - also have done it in guitar stores (with their permission) to see what something sounds like with fresher strings!!