Protect it from the sun:

UV rays in sunlight are a major cause of skin ageing and skin cancer. Protect skin from sun damage by avoiding direct sunlight on sunny days or in hot countries, and by using sunscreen.

Stop smoking: Smoking causes skin to age prematurely by breaking down collagen, which reduces its elasticity, and cutting off supplies of oxygen and nutrients.

Drink less alcohol:

Drinking alcohol can dehydrate your skin, leaving it looking tired and old. Drink alcohol in responsible amounts and stay hydrated.

Wash regularly:

Wash skin regularly using mild soap and warm water, but too much washing or harsh soaps can dry skin out. Moisturise after washing if you have dry skin.

Source: NHS Choices