I swear by Boots number 7 Protect and Perfect range. I use their serum (the intense one as I am older) their eye cream, their day and night cream, all intense. But most important is to always cleanse off makeup; never go to sleep with it on, and moisturize day and night.
But if your face is going saggy due to weight loss and if you can afford it, look for a beautician that does microcurrent facials, otherwise known as "non surgical facelifts". The beautician applies gentle electrical current to the facial muscles that helps to lift them and strengthen the underlying structure. It's like slendertone does for your body, only this is for the face. I have this facial every two weeks, and the lady who does it has strict instructions by me that if she notices my face sagging, to tell me and we'll do it more often.