Loose or poorly adjusted saddle
Proper saddle position is often a matter of personal preference. Saddles can be adjusted for height, tilt and fore/aft position. The exact combination that works best for you will depend on your physical size and your riding style. For more information about checking saddle position and making adjustments, check elsewhere in the site. Make sure your saddle is secure before every ride. Most saddles are held in place by a few simple nuts and bolts. All of them should be tight enough to resist vigorous shaking.
An over-extended seat post
This is a serious safety hazard. In general, at least two inches of your seat post should be inserted into your frame at all times. This rule however will vary considerably if you follow the growing fashion of "showing a lot of seatpost". If you have to raise your seat post beyond its extension limit line to get comfortable on your saddle, it's probably time for a larger seat post, or a bigger bike. Once you've found the "perfect" saddle position for you, mark your seat post and your saddle rails with tape or felt tip pen (indelible) so you can readjust them easily.
Regularly remove your seat post from your frame and coat it with a thin layer of grease before re-installing it. This grease layer will help protect the post against rust and corrosion and more important, prevent the post seizing in your frame.
Checking your saddle
You should check your saddle to ensure it's secure and properly positioned. Grasp it firmly and attempt to move it out of position while holding your bike steady. Some side-to-side movement will probably occur but if your seat post shifts up and down, or your saddle feels loose, make adjustments. Also check your seat post visually to make sure you haven't exceeded the seat post extension limit line (the furthest point that the post can be safely extended upwards) has not been exceeded. This is clearly marked on the side of your seat post. Regularly remove your seat post from your frame and coat it with a thin layer of grease before re-installing it. This grease layer will help protect the post against rust and corrosion and more important prevent the post seizing in your frame.
Saddle Cleaning Procedures
To keep your saddle in good condition, simply wipe it down from time to time and treat it with UV-protective conditioner. Most models can be cleaned with light soap and a little clean water. Others require special cleaners designed for their specific materials.