For me it's void meditation - just sitting still, spine erect, and watching your breathing.
Posture: I use the 'god position' (just sitting on a hard chair with my hands on my thighs, no mudra or anything, without touching the backrest!). Jason Miller thinks that posture is too unstable and recommends sitting on the floor with crossed legs. I admit that he may be right because my pelvis started to wobble when I first started out but then I chanced upon this trick with the cushion (see p. 3), and now it's fine.
I confess it took me some months to find the correct posture. I tend to slump forward, and somebody somewhere even wrote that he first had to develop the right muscles in his back to sit upright because he was so used to lounging around (there may be something to it). Anyway, when I think of the ideal posture, the Egyptian statues at Abu Simbel are as good as fixed in my mind:
Now it's beginning to feel like the correct posture is locked in place whenever I practice although I still make some small adjustments now and then.
Not-thinking: probably one of the hardest things you can do (or not do, as the case may be) with your mind.
"While motionless and breathing deeply, begin to withdraw the mind from any thoughts which arise. The attempt to do this inevitably reveals the mind to be a raging tempest of activity. Only the greatest determination can win even a few seconds of mental silence, but even this is quite a triumph." (Peter J. Carroll, "Liber Null & Psychonaut"). Spot on. Let intruding thoughts come and go, let that tempest blow itself out and don't berate yourself for your initial inability to still the mind.
I once read somewhere that meditating meant "ceasing to struggle". That raging tempest Carroll speaks of? Let it rage, it will eventually calm down. Noises from outside your room? Most human environments produce noises, you're not meditating in the basement, are you? Worries? Accept them but here is a chance to take some time out from brooding if you want to. So I try ceasing to struggle (and ceasing to struggle with ceasing to struggle, etc. as well) for half an hour or so.