One of the most important aspects of a sitting meditation is to be able to sit comfortably, without an aching back, or legs hurting or going to sleep. If you are in pain or great discomfort, the only thing you will be meditating on is that!
Sitting Positions
Options for sitting are in a chair, or on a meditation bench, or on a pillow on the floor. Most westerners are not trained from birth to sit comfortably on a hard floor. So a chair is probably best for most of us, beginners or otherwise — and many very great meditators with many years experience use a chair or stool for their meditations. It is not a sign of lack of meditative ability if you are unable to sit in the lotus posture or any other floor sitting position.
In a Chair
Get a fairly straight-backed chair and sit forward in the chair so that both feet are flat on the floor. If your feet do not touch the floor, get a shorter chair or place a pillow or two under your feet to raise them so that your thighs are parallel to the floor. Do not lean against the back of the chair! The idea is to sit with an upright, unsupported spine.
However, if you are not used to sitting this way, or if you have weak neck / back muscles or injuries, there are ways to overcome this challenge. Get a firm pillow of some sort (the crescent shaped ones are very good for this) and put it between your back and the back of the chair. The feeling you want is that of support, but not leaning into it. Move the pillow around until you achieve this feeling. If you want to place a pillow in the seat of the chair, to cushion a too hard surface, that is fine.
Meditate for short periods of time in the beginning and work up to longer amounts of time. This way your back muscles will strengthen gradually. Yoga stretches and other such exercises also strengthen your back muscles with time and regular practice.
On the Floor
Meditation benches are a wonderful invention for making the legs feel comfortable and un-pressured and keeping the spine up-right. Finding the right size and height is important. Padding on the seat often helps. Adding small pillows under the knees or ankles might facilitate your comfort also.
If you have never tried a bench, please be sure to experiment with one. Some people are more comfortable sitting cross-legged on a pillow. The crescent-shaped or round-plump pillows are designed to help with this position.
Experiment!
Have a chair, lots of pillows, a bench and whatever else you want to try. When one position becomes tiresome, calmly switch to another. Eventually you’ll find the best one for your body-type. Remember everybody’s body is different.
A Blanket or Two
Many yogis recommend sitting on a wool rug, blanket, or piece of silk. Also the place you meditate should be a little on the cool side with a source of fresh air if possible. Thus another blanket or warm meditation shawl should be handy to wrap up in. The body does cool down a bit when you sit still for a while, so a wrap is often important to maintain an even body temperature. Get comfortable, but stay awake and ready!
Martin Boroson has developed a simple and effective method to access a balanced inner state in only one minute: One-Moment Meditation. The following video explains this method and proposes a real-time exercise.
Meditation is never the control of the body. There is no actual division between the organism and the mind. The brain, the nervous system and the thing we call the mind are one, indivisible. It is the natural act of meditation that brings about the harmonious movement of the whole. To divide the body from the mind and to control the body with intellectual decisions is to bring about contradiction, from which arise various forms of struggle, conflict and resistance. 
There are some attitudes that are especially powerful in hindering the positive and beneficial effects of meditation that I will address here, since it seems that far too often we fall into these thought-patterns that result in us having a sub-par meditation experience with sub-par results.