Seatposts must be sized very accurately: the 30 different common sizes come in increments of 0.2mm. One size too big won‘t fit your frame; one size too small will fit but rock slightly at every pedal stroke, slowly destroying your frame. If you have your old seatpost, the right size is stamped on it. If in doubt, get your bike measured at the shop.
All seatposts have a minimum insertion line. This is usually indicated by a row of vertical lines printed or stamped near the bottom of the seatpost. The vertical lines must always be inside the frame. If you have to lift your seatpost high enough to see the marks, you need either a longer seatpost or a bigger bike. In the unlikely event that you have no markings on your seatpost, you need a length at least 2.5 times the diameter of the post inside the frame. Seatposts that are raised too high will snap your frame.
Adjusting Saddle position
Step 1: This is the most common type of saddle fixing. The saddle rails are clamped between two plates with a single bolt. The bottom of the lower plate is curved to match the top of the post. Loosening the bolt (6mm Allen key) allows you to slide the saddle backwards and forwards or to roll it to change the angle. Start with the top of the saddle horizontal, clamped in the middle of the rails. Remove and regrease the fixing bolt regularly.
Step 2: This design allows you to control the angle of the saddle precisely. To tip the saddle nose downwards, loosen the back bolt slightly and tighten the front bolt firmly, one turn at a time. To lift the nose, loosen the front bolt one turn and tighten the back bolt. To slide the saddle along on the rails, loosen both bolts equally, reposition the saddle, then retighten the bolts equally. A ball-ended Allen key is handy here, as the front bolt can be tricky to access.
Step 3: This design has two small Allen keys at the back of the clamp. Loosen both to slide the saddle rails in the clamp or to roll the clamp over the curved top of the post.
Step 1: Saddle angle is critical for a comfortable ride. Pedalling in this position, with the nose of the saddle tipped upward, will push you off the back of the saddle, lifting the front wheel off the ground when climbing. For hardtails – sitting on the bike compresses the fork, dropping the saddle angle – you‘ll have to start with a slight rise so the angle levels off with your weight on it.
Step 2: This position can help relieve the discomfort of a saddle that doesn‘t suit you, but it tips you forward toward the bars, causing wrist and shoulder pain. This position is also often a sign that your saddle is too high – try levelling it off and dropping your seatpost a few millimetres into the frame.
Step 3: A level saddle position is always the best starting point for full suspension bikes.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Fitting a new stem”