Texture-wise, we're best off choosing a concealer that's suited to our skin type. If your eye contour is dry or dehydrated, opt for a liquid texture that will be both easier to apply and moisturise this zone, without harming your already fragile epidermis. Those of you with mature skin should avoid drier concealers as they'll just sink into and exaggerate wrinkles and crow's feet. However, hydrating formulae should be avoided if you have oily or combination skin as well as eyelids that tend to become greasy during the day. If that's the case, go for a stick concealer that will stay put.
If your dark circles aren't too pronounced - a bit blue or pink - a fine, fluid texture will disguise them. Concealing serums or light-reflecting concealers are great as they'll leave you looking fresh-faced. On the other hand if your dark circles are pronounced or brown, apply a cream concealer or stick that will cover them better and not budge an inch. Whatever you choose, don't apply too much! It'll only make your dark circles worse and leave you looking even more exhausted.
There are loads of tones to choose from and the trick is to not go by the colour of your skin. It's the colour of your dark circles that counts. Bluish dark circles can be zapped by a pink concealer (a red hue), whilst violety ones can be covered by beige concealers (a yellow hue). This is because complementary colours (those that face each other on the colour wheel) can effectively mask each other.
Concealers can be applied before or after your foundation - it just depends on the result you want. For a more natural look, apply it before your foundation, particularly if it's a powder. If your dark circles need a thicker coat, apply it after your foundation. The best way to apply a concealer is to use your fingertip or a brush. Gently dab it on, working out from the inner corner of your eye. Smooth it across your dark circle and then up to your lower lashes.