What do you mean for "stability"; thermal stability, mechanical durability, environmental stress cracking resistance, fatigue resistance, stability in color, UV resistance, resistance to specific chemicals...? I would like to remind you that only the term of stability does refers to nothing. But you are lucky on the fact that polyolefins are probably the most stable thermoplastics, regarding the issues listed above and exhibit the longest shelf life due to their chemical structures and formulation features. They include many specific additives to increase their stability and make processing operations easy. Manipulation in (any) stability is generally job of manufacturers.

I agree with Ali's answer above - there's too little data provided to give you a specific question.

A general answer would be one of methodology:

If you want to increasethe shelf life of such bottles, identify the phenomenon / reaction that limits it at the moment (what happens to the bottles after prolonged storage to make them unusable?). Once the phenomenon is identified, investigate it in detail and find factors affecting it. Afterwards, it is only a matter of optimisation and fine-tuning.

You may want to check this related RG discussion: https://www.researchgate.net/post/What_is_life_spend_for_low_density_polyethylene