Although you can use a chair to meditate on, it's not as satisfying as sitting on the floor. Somehow, being on the floor gives a more "grounded" feeling.
Finding good cushions is important. They need to be really firm, and most pillows just compress too much and can't give you enough support. The same goes for most ordinary, household cushions. However, I have a lovely buckwheat pillow that is perfect when I turn it on end.
This meditator is using cushions (called zafus), that are specially designed for meditation. He's kneeling with them between his legs. Most people who sit astride cushions need two or three, depending on the height required.
The important thing is to get the right height. If you sit too low, you'll end up slumping. Slumping interferes with your ability to stay aware, and can lead to discomfort.
If you sit too high, then you will have too much of a hollow in your back, which can lead to pinching. When your back is relatively upright, without you having to use any effort to keep it that way, then you've got the height about right.
Although the meditator above has his hands resting on his thighs, I recommend having your hands supported in front of you. You can either have another cushion in front of you to rest your hands on, or you can tie something round your waist and rest your hands on that. I've used a sweater with the arms tied behind my back. If you arrange the sweater carefully, you can make a little "nest" for your hands to rest on. A blanket can also be used to provide support for your hands. Tie the blanket fairly tightly round your waist so that it covers your legs. Then arrange the blanket so that it provides a little "ledge" that you can rest your hands on.
