I L-O-V-E liquid eye liner. I was doing the flick way before Amy Winehouse came on the scene and it crept onto the catwalk (remember Chanel's models with their dreamy fly-away beehives and thickly lined feline eyes back in 2007? It's pictured below in case you don't!). As with mascara application, I am a firm believer that anyone can learn to do great make-up if they familiarise themselves with their facial features. I first started experimenting with eye liner when I was about 13/14 years old because I had to do stage make-up for ballet performances. It was a disaster so I bought one pink and one lilac kohl pencil eye liner from MAC to practise with. These were both beautiful with my green eyes and were much better to play around with as mistakes weren't so obvious.
I did not master the art of applying liquid eye liner until a fellow dancer told me her technique (which I explain below). Since then, I have often gone back to the classic flick for a night out. I have stayed loyal to Rimmel's liquid eye liners (it was the first I tried) but also use Shiseido's in grey. A friend uses YSL's so I can vouch for theirs too. I do not think this is something you need to splash out on, though.
Do test before you purchase by drawing a line across the back of your hand (not on your face - you don't know where it's been!!). The line should be opaque and glossy. Many will come out grainy, blotchy or too dilute. I also recommend that you get an eye liner with an applicator like a felt-tip and not a brush. These are much easier to handle and, because of their tapered shape, allow you to draw a better line and control the thickness.