Genetics, unfortunately, are 90% of "good skin" IMO (a bit like weight really).
I'm lucky, my mum had beautiful skin, my nanna had beautiful skin - both didn't start to show wrinkles until way into their 40's.
Still, just like the genetic predisposition to weight gain, there are definitely things you can do to improve your fate skin-wise.
Omega 3 and 6 oils as mentioned before are the best way to internally care for your skin. Take an Omega supplement if you can't eat enough oily fish, avocado and nuts - these oils moisturise your skin from within and improve the cellular structure and elasticity. Omega 3 oils (fish and flaxseed) also have the added benefit of calming inflammation -perfect for psoriasis or eczema sufferers. They also help heal acne scars.
Water is most definitely right up there with Omega fatty acids as one of the most important skin helpers. If you drink no water and only soft drink and coffee/tea, you're probably committing the greatest skin sin; dehydration is a killer.
STAY OUT OF THE SUN! The cardinal sin of skin "don't"'s is sunbaking - it will wrinkle and damage your skin horribly. I've heard some skin experts say that the best beauty secret they can ever divulge is to wear a 15+ sunscreen every day (including in winter).
Ditto with smoking. Yeah - I smoke, I can't judge

... but it will create little lines around your eyes and lips. Stress and lack of sleep will also wreak havoc on your complexion.
finally I'd say invest in a good moisturiser (guys too) at least from the age of 25 onwards. By "good" that doesn't mean top-notch brands, because you're buying the brand name and advertising as well. But a middle-priced, reputable brand which has hydrating effect - like with vitamins C and E, is a good investment. Use at night, and use a lighter less expensive brand in the morning. It just helps create a moisture barrier, if nothing else, so that the environment around you does not suck out the moisture in your skin.
Later down the track, some alpha-hydroxy acids and so on in your moisturisers can help lift those dead layers of skin so that the ingredients get down further... I also think that exfoiliation is great for this purpose.
But yeah, in terms of importance:
1. Sunscreen
2. Water
3. Omega 3 & 6 fatty acids
4. A good moisturiser