I don't have big boobs, but I have been exercising my pec muscles since I was 17 to prevent sagging. I don't use weights, just flex and unflex while watching TV and stuff.
I also have used Neutrogena Sesame Seed oil on them since then too. I always doit right after the shower, and ONLY on my boobs. A little goes a long way, and I do it a couple of hours before work so I am not treacherous. It may or may not help, but to me it seems to, as my boobs haven't changed since I was 16 or so and are super silky.

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