One of the ways I sometimes "cheat" in this area is by taking advantage of my Tablet PC. I use Microsoft OneNote as my "blank pad" and create my low-fidelity prototype directly on the screen with my tablet pen. Although I really enjoy working with pencil and paper (and crayons, markers, etc.), I find the tablet solution to be a good one when I'm pressed for time. If I want to make changes, I can simply erase an area of the screen or create a copy of the current page and make the modifications on the new version. Taking advantage of OneNote allows me to group related prototype pages by project, perform searches on keywords, backup my work easily, etc.
Using OneNote on the tablet is also a great help to me when I'm further along in the design process. I can use the program to perform a quick screen capture, then use digital ink to annotate the changes needed directly on a picture of the screen in question. This aids me greatly in remembering what we were doing if I go back to find some details in the future.
Another promising tool for quick "low-fidelity" prototyping is the DENIM project. DENIM allows you to create actual working prototypes that maintain the spirit of quick, iterative low-fidelity methods. With this tool, you draw your prototype as you normally would. The cool part is that the system is very smart and can identify common design patters, such as text boxes, labels, radio buttons, etc.
Check out the videos on the project site to see it in action. I think you'll agree it has some merit. I have to admit that I haven't tried DENIM yet, but it's certainly on my to do list. If you check it out, let me know what you think.
I'm sure there are other creative developers out there that use electronic tools to perform quick and dirty prototyping. If you've got some ideas you'd like to share, please post them in the comments. No matter how you go about it, the important thing is that you prototype often and early. This will help you deliver better systems to your users!
Wow...just realised as I was saving that this is my 100th post. Didn't think I'd make it this long, especially since I sometime get a bit long-winded! :-D Thanks for sticking around everyone!!!