[kiddietalkingaboutairliners.net]I think it's experience. The theory is that the nosewheel follows the yellow line and the plane ends up in the right place.

That said there is a guidance system as well - the lighting thing on the wall which shows the pilot whether he's bang on, a bit left, or a bit right. The guidance system only really kicks in for the last few yards.

The difficulty in practice is that the cockpit isn't always slap bang above the nose wheel so you have to 'over-run' (so to speak). It was a particular issue with Concorde (just have a look at any phote).

I've seen planes get out of shape a few times. It's not unknown for them to end up needing a bit of pushing and pulling.[/kiddietalkingaboutairliners.net]