Hmm, I was going to ask about sunscreen, but you answered that in your update. Well, at 40 our hormones really do start to change and everyone starts to go through the "change" at varying times. My Mom started peri-menopaus in her early 40's. Hormonal changes are a big factor in women losing collagen (skin elacticity), less hydration, etc.
I have a friend who goes to a homeopathic hormonal person (Dr?) where all of her hormones were tested, and then she takes various types of vitamins and hormone type therapy to keep her hormone level up as close to her current state. In other words, she won't go through menopause and will always have her period (as long as she is taking these hormones). Something like that would probably help and is more natural than something like botox.
Other than something extreme like hormone therapy or botox, I say start embracing it;) The fact is that at 40, all sorts of things start to change. I turned 40 this year too and have noticed little differences here and there. I spend slightly more on my eye creams and moisturizers now and less on cocktails & nights out;)
The best thing you can do for yourself is continue your healthy lifestyle, get lots of sleep and drink lots of water. Oh, and give up any sodas & caffeine (max 1 cup coffee/tea/day) - those will wreak havoc on your skin.