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.
To start, you want to figure out the desired dimensions of your desk, including what heights you want the desk's surface to be at when you're standing and when you're sitting.
Based on various resources on the internet, such as this one, regarding the best ergonomics for your computer workstation, the general rule is you want your keyboard to be roughly at same the height as the bottoms of your elbows when you've got your hands on the keyboard and your upper arms hanging relaxed by your sides. You can easily measure this distance with a tape measure.
My desk had to go up and down by 17" to be usable for both standing and sitting. This is convenient because 18" is a common size for drawer slides.
In the sitting position, the desktop needed to be about 25" above the floor for me, while in the standing position, it had to be about 42" high. Because I wanted a 3" backstop on the desk, and I also needed the paracord connecting the desktop and counterweight to be suspended above both objects via pulleys, 48" turned out to be a convenient choice for the uprights on the desk's frame.
You should determine what footprint you want the desk to fit into and how large a desk surface you want to work on. Note that if your desktop sticks out too far, you may need to design better bracing for its underside than what I used (which was sturdy shelf brackets.) There could be a lot of torque on the front edge of your desktop.
My desk fits into a total footprint of 24" deep by 32" wide, which I needed because of the space it has to fit into in my home. The desktop is only about 15" deep and 32" wide. Obviously, I don't have a computer tower sitting on my desktop -- there's no room! Instead, I have my laptop on the monitor pedestal and a peripheral keyboard below it.
Keep in mind that, for this sort of design, you have fewer options for storing things under your desk because there are moving parts down there.