How to sit for meditation or pranayama practice

When choosing a posture for sitting meditation, two important things have to be taken into consideration.
The first one is comfort, as you will be staying in the posture for a while and you do not want to have to shift your posture constantly (in fact, you do not want to have to change your posture as all, as this will disturb your concentration). Also make sure the breath can flow freely and is not constrained by your clothing or by your posture. You should feel at comfortable and at ease, with no tension in any part of the body.
The second thing to consider is that your spine should be straight and vertical.

If possible, sit cross legged on a meditation cushion or a zafu to make the knees lower than the hips.
If this is too difficult, it is also possible to kneel (possibly using a meditation stool or a zafu to ease the pressure on the knees and ankles). If this isn't comfortable either, sitting on a chair is a good and inexpensive option, especially for people with stiff hips. Make sure you don't lean back against the back rest, but rather sit upright with the spine straight and vertical.
Do not lie down as you are likely to fall asleep.