Eyeliners... I personally think it's the trickiest thing to apply when it comes to makeup but also makes the biggest dramatic impact from looking dull and bleh to polished and well, awake! :P
If you're looking for some helpful tips, here's a great video by YouTuber MakeupbyTiffanyD that breaks it all down for you from using pencil eyeliners to liquid liners to gel and cream-based liners. (In general, I'm a big fan of all of her make-up videos so if you like this, you can see more on her YouTube channel.)
For my upper lashline, I personally like using the gel and cream-based liners and I bounce around between ones from Bobbi Brown, MAC, and Clinique. I think I like the ones from Bobbi Brown the best because they seem to glide on easier and are rich in color. (They're also the most expensive, too, unfortunately.) As for brushes, I used to use the MAC 266 angled eyeliner brush but am now using a teardrop paint brush I got from Michael's for $3-4. I haven't tried the bent brush like the one used in the video so I may give that a try as well.
For my lower lashline, I always use a pencil eyeliner, typically one of the MAC Powerpoint pencils but I like the ones from Urban Decay as well.
What are your favorite eyeliner techniques?