Quote (apenpaap)
but you have to press F every now and then to let your ship move down with the curve of the planet, as the lazy planet won't pull you down itself.
Actually you can orbit properly, but you must be in 'gameplay' (4 key) mode. It helps to know the orbital velocity for your current altitude, so let's use Earth as an example. At 350 km above the Earth, you can orbit if you fly at about 7.7 km/second parallel with the ground.
Without a HUD it can be hard to orient properly, but press 1 for Free Flight mode, then shift-g and land. After your camera is oriented with the ground, translate straight up without rotating until you reach 350km. Press ctrl-3 to access the ship menu, choose one, and then click Teleport Here. You'll see it appear and immediately begin to fall away from you. Click Switch To and now you're flying the ship, in the same orientation as your camera. Accelerate forward until your speed is about 7.7km/sec. Visually ascertain whether your front and back velocity markers are an equal height above the planet, and adust your speed accordingly with use of the translational thrusters.
It's not ideal, but that should get you started.
ENG
New site
Netherlands 
Pirate 
Russian Federation 
Antarctica 
United States 
Interplanetary flight might be a little easier in some systems - like Gliese 581 or Kepler-11 - that have tightly-spaced planets. In Kepler-11 you would need a rather high delta-V though. 
There was no game mode or ships back then. XD