I found myself needing to texture two walls in a basement room and not wanting to get a sprayer to splatter drywall mud everywhere. What I came up with was a very fast, efficient way to achieve a similar effect without renting any equipment.
Assume that the walls are smooth to your satisfaction, i.e. you have taped and sanded the seams, filled in the screw holes, etc.
1 box of drywall mud
8 inch drywall knife
Step 1: Water down the Mud
I watered it down until it was about as runny as clam chowder. Just keep adding about a large handful in a bucket, then stir in water.
Rather than use a heavy mixer bit and drill, I just used a wire whisk from the kitchen.
Step 2: Trowel it on
Now use the Five Fingered Trowel method to apply the soupy mud to the wall. Spread it back and forth. I found I could do about thirty square feet at a time without the first part drying out before I was done with the last. As soon as a section of wall was covered in mud, I used my trusty Target bag, all balled up, and jammed it into the wall. This part took me a few tries. Initially I was kind of dragging the bag around, but what you want is to push the bag into the mud, then pull it straight back out so that you create peaks in the mud.
When you're done, the section of wall you just did should look peaked like a cupcake.
After a few moments of drying time, the thicker mud, closer to the wall, will be dry, while the thinner high peaks will still be wet.