Sitting and meditating comfortably for long periods of time is hugely affected by having the correct posture during meditation. I’ve noticed that in my personal practice, having the correct posture and positioning has a large effect of each session. It is something I have to constantly readjust and re-examine on a daily basis.
One of the best ways to sit during meditation is to place one self in the 7 point meditation posture, also known as the 7 point meditation posture of Buddha Vairochana. In this interesting intruction on meditation, Gehlek Rinpoche explains how to use the 7 point meditation posture
The meditation posture
The posture we recommend for meditation is the seven-point posture of Buddha Vairochana. Sit on a comfortable cushion in the vajra posture with both legs crossed and your soles upturned. Indians call this the lotus posture; Tibetans call it the vajra posture. It is the first of the seven features of the Vairochana posture. If you find this or any of the other points difficult, simply sit as is most convenient and comfortable.
The seven-point posture is actually the most effective position for meditation once you develop familiarity and comfort with it, but until then, if one of the points is too difficult you can substitute it with something more within your reach.
Keep your back straight and tilt your head slightly forward with your eyes cast down along the line of your nose. If your eyes are cast too high, mental wandering is facilitated; if too low, sleepiness or depression too easily set in. Don’t close your eyes but look down along the line of your nose to an imaginary point about five feet in front of you. In order not to be distracted by environmental objects, many meditators sit facing a blank wall. Keep your shoulders level, your teeth lightly closed and place the tip of the tongue against the front of your hard palate just behind your top teeth, which will prevent you from getting thirsty when engaging in prolonged meditation.
And for some inspiration and guidance on working with the mind during the meditation session, Mingyur Rinpoche gives some advice to beginners: