We used a very effective cascade elevator in 2011. The main fixed mast was 2 pieces of 2 x 1 x 1/8 channel with Igus linear slides mounted inside. The first stage was 1 1/2 x 1 x 1/8 rectangular aluminum tube with the igus shuttles mounted. It was driven up by a Gates belt driven by a 775 motor and a Bane Bots 64:1 gearbox. It was helped down by a constant force spring. The second stage also rode on Igus linear slides and was pulled up and down by lightweight cable. Once we got it fine-tuned, it worked really well.
http://www.othsrobotics.ca/images/hank.jpg