1. Use an applicator with a felt tip. The brush tips are unwieldy and make it difficult to vary line thickness and make points. They're also flimsy. The felt ones are more stable and can offer an even line.
2. Take your time. When applying, do it in little brush strokes as opposed to trying to get the perfect line all in one go. It's possible, but one eye twitch and you've smudged it. You're more likely to get an even line if you do it in increments. You can also go back and carefully correct any unevenness once you're done. Just make sure not to go overboard in the course correcting.
3. Symmetry is for suckers. Just as long as you don't have just one eye with a long flourish at the end. No one is going to notice if one spot is slightly thicker than the other or if one side has a finer point.
4. It's okay to make mistakes. For the longest time my eye would twitch whenever the cold liquid hit my lid. It's cool just have a q-tip and some make-up remover handy.
5. Colour can be your friend. Black is fun for a dramatic edge, but a dark brown is a great subtle look for the day time if you're timid. When you're feeling bold, purple, blue and green are great alternatives.
6. Do not put it on your lower lid. From experience I've been told it's too much. Use a pencil instead.
7. But also don't be afraid to experiment. Cat-eye is the most common liquid look, but that doesn't mean it's the only way you have to wear it. Make sure you have fun with it. It is just make-up in the end. If you can't wash it off you've done something wrong.
With love and cupcakes
Cathryn
*edit: As for brands, I like to use a Canadian kind called Annabelle which is available at Shoppers. Other than that, just try to find something with a felt tip and you can't go wrong.