Anyways since I always wear a nude lipstick I kind of need to wear a lip liner anyways as I find with out it your lips can just blend into you skin colour very easily. The liner not only makes your lips look bigger but it just adds that needed definition you need with a nude lip.
My favourite lip liner by far is 'Willow' by Wet n Wild. I swear by it. Its has such an easy application, its so cheap (€2/€3), and it blends in extremely well with so many nude colours. So I can use this one lip liner with many different nude lipstick colours. Also another fab thing about this lip liner is that its super long. Much longer then the majority of lip liners, so it lasts for ages.
(I tryed to get a picture of a swatch of the colour but it doesnt do it justice, so your just going to have to trust me.)
Another colour by Wet n Wild I love is 'Chestnut' this also can be worn with nude lipsticks it just makes the colour darker. I always make sure I have at least one chestnut lip liner in my bag. Unlike willow I usually stock up on that colour.