How to do a track stand?
Here are my snippets of advice from when I learnt to trackstand:


Start off by practising on a slight uphill. This way you only need to practice the forward pressure part of the movement. As you get better you can move on to smaller and smaller slopes. In urban riding, you can often use the camber of the road as your slope.
Use the right gear. Not really an issue with a fixie, but on a bike with gears you want a gear that is in about the middle of the range. Too low a gear will give you too much leverage and you will tend to over correct. Too high a gear and you won't have enough leverage to move the bike easily.
I find it easier to track stand standing up, pedals horizontal with my good foot forward and with the saddle braced between my thighs. Later you can progress to seated trackstands.
If the bike starts to lean towards the direction that you've turned your front wheel, then a slight pressure on your leading foot should shift the bike back under you.
If the bike starts to lean in the opposite direction, then you have to ease off the pressure on your front foot and let the bike roll back down the hill slightly. This is probably the trickiest part to get the hang of as it is a fairly unnatural backwards motion at first, but once you get the hang of it your trackstands will progress well.