Saddles And Seat Posts - Mountain Bike Zen
Adjusting Seat Height
Fairly straight forward - loosen the quick release or the bolt depending on what you have on your bike, and then raise or lower the seat to the correct height and tighten up the bolt or quick release again. You want to adjust the seat height so that when your foot is farthest from you with you pedal in the lowest position your leg is almost straight but still with a little bend. If you are just starting out you may need the seat a little lower until you have built up a little more muscle strength.
Adjusting Mountain Bike Saddle
Soemthing alot of people don't know about is that the saddle can be moved backwards and forward as well as up or down. Usually there are two allen key bolts holding the saddle rails in place - loosen these and you can slide the saddle backwards and forwards. This will give you some ability to adjust how close you are to the steering bars and the position of your leg of the saddle when riding in a seated position. Generally your knees should be over the pedals and you should feel comfortable - if you are constantly shifting in your seat when you first mount the bike think about which way you are going and see if you can adjust your saddle to suit.
Suspension Seat Posts
You can fit a suspension seat post which will make for a more comfortable ride and is a great upgrade for a hardtail mtb but can be used for extra shock absorption on a full sus. These can be built like a front suspension coil fork with a spring inside the tube so bounce on, or they can have a set up with an elastomer which gets squished with the shocks a bit like a big elastic cork.
For extra comfort when riding you can also choose a sprung saddle which will have a little bounce and a gel saddle or gel pad for the saddle which gives a soft jelly like layer that will mould to you behind and absorb a little of any impacts too.
These extra bits and bobs can help comfort and also reduce the chance of musculo skeletal injury if set up and used properly. For the real hard ass's you don't need any of these.