A good quality seat post will save weight and hold your saddle securely. Seatposts are made in various materials, alloy and carbon being the two most common, alloy posts are lightweight, durable and cost efficient, while carbon posts are the popular choice for top end machines. Seat posts vary in where they hold the saddle relative to their own centre line so make sure you have enough adjustment on your saddle rails to allow for this or you could end up with your saddle too far forwards. A seat post needs to fit your frame exactly and are measured by as little as 0.2mm. When choosing a new one, either read off the measurement from your old seat post or measure it carefully with a vernier calliper. Make sure you lubricate the frame (unless they're carbon fibre) before insertion or you'll struggle to get it out again.