You don't need to buy a whole new desk to start working standing up. In fact, I think it's more flexible and attractive to use an add-on standing desk setup, since you can move it anywhere and put it away after work is finished for the day. Here's one you can make yourself using free plans with metal piping, a common wall shelf, and a standard VESA bracket for a monitor.
Designer and builder, Nate Bunnyfield:
This is my first time doing anything with Kee Klamps and second standing desk design. It took me about a week to choose what to make and one night to build it. I commissioned the reclaimed wood top from the guys at squarenailchicago.com for $30.
The Sled Desk components list:
If you already use free 3D design program SketchUp, you can download Nate's Sled Desk plans to emulate (or customize) for the nice price of $0 here. And the complete project with more step by step images and details over at SimplifiedBuilding.com.
(Images: Nate Bunnyfield)