Many people will say a good nights sleep lol ........ but there can be other factors. I have an illness that causes sleep deprivation and I know another model who has ME (or in basic terms chronic fatigue), so I used to have horrible dark circles under my eyes, whilst a light reflective concealor under the eyes helps (I used to use max factor - worked well for me), theres something Ive found recently that works wonders on my dark circles, I use garnier under eye roll on daily in the mornings (youthful radience range I think about £7 - 10 in boots or superdrug), its practically eliminated the dark circles I used to have =)
I swear by Estee Lauder 'disappear'. For me it works better than touche éclat, and is more versatile as it is a concealer too. Touche éclat shouldn't be used on blemishes as it has light reflecting particles which can actually draw attention to the area.
Touche Eclat is effectively a highlighter but many do use it under the eyes as it brightens and does hide the darkness. You should use a normal concealer for blemishes etc. I swear by Touche Eclat and I use it on every job I get for photoshoots and also weddings. It's £24.50 but its worth it as you only need one click for one half of the face to highlight features. Helen Douthwaite MUA