The classical "praying mantis" position.
There are three commonly-adopted postures - kneeling, sitting or standing - but there may be circumstances in which you have little choice. If you are confined to bed (or even merely in a wheelchair), you must make the best of the situation. If you are driving a car, and feel a prayer coming on, then you will probably lose control of the car should you decide to kneel down or stand up. In this case, prayers were probably appropriate.
If your plane is coming into land, make your prayers while seated with your belt fastened.
Brace! Brace! Oh, and say your prayers.
In church, the traditional posture for prayer is "meekly kneeling on your knees". In modern churches this is not always possible: there is often not enough space in which to do this, and the comfy cushions have been removed. Well, everyone knows the expression "It's as difficult as finding a kneeler or a hassock." [Pun. Not a very good one, sorry.]
This is where all the hassocks ended up.
"At the name of Jesus, every knee shall bow." Well, so we thought, but nowadays - at least in Catholic churches - the modern thing is to stand up instead. Or, if we're being really lazy, "We'll remain seated for some prayers."
Traditionalists, of course, will kneel even if there is no hassock. Some will even bring broken glass with them, to make the experience more uncomfortable, and to mortify the flesh. Monks and nuns kneel on cold stone floors - well, they used to, certainly.
Prayer while levitating. Comfortable, but difficult.
Of course, prayer does not simply involve saying words - whether made up on the spot or already written down (this will be discussed in a future instalment); one may count hymn-singing and liturgical dancing (oh dear) as forms of prayer. In that case, the correct posture should still be reverent.
This is not the correct posture for prayer.
No, on balance, kneeling is more respectful than standing, standing is more respectful than sitting, and cavorting around like something from a circus is worst of all. But who am I to judge?
A cartwheeling priest in Westminster Abbey. A prayer of thanksgiving.
For prayer you should probably close your eyes (unless you are driving a car, doing a liturgical cartwheel, reading the prayers, etc.) It is considered rude to stare at your neighbours, although everyone seems to do it. Er, so they say. How would I know that?
You can clasp your hands together in various holy-looking ways (DON'T put them in your pockets). In Evangelical churches it is usual to wave them in the air so that God will notice you more easily.
Finally: how to pray in an ecumenical service.