There's evidence that sitting for long periods of time is bad for your health. However, standing for long periods is also not great for your body, and trying to switch from a sit-down desk to a standing desk too abruptly can result in a sore back and aching feet.
So although it would be healthier for us all to use standing desks, ideally, you'd like to be able to ease into it gradually -- and hopefully without needing to schlep your computer back and forth between two desks.
To accomplish this, I designed and built a convertible standing/sitting desk. My computer stays put, and I can smoothly and easily adjust my desk up and down to either stand or sit in front of it.
This desk is designed like an elevator. There is a counterweight at the back that is attached to the desktop, and both the desktop and the counterweight move up and down on drawer slides that act as guiding rails.
In this project, I kept things very simple. I bought my supplies at Home Depot and had them "rip" my wood, i.e. cut it to the correct sizes for me (you pay a little extra for this.) Then all I had to do was drill my guide holes and bolt everything together like The Thing That Came From Ikea.
The entire project cost $200, not including my local sales tax. The equipment list is in the last step of this Instructable.
By the way, if you've got modest carpentry skills, you aren't obligated to birth a pseudo-Ikea-monster like I did. You could certainly make a more permanent (and attractive) version of this desk using wood glue and proper joining. You could also scale this desk up to a larger size, because as long as your desktop platform is braced adequately, you can throw a sock full of buckshot in the counterweight to lift a heavier desk and computer.