Here are some tips I use to get some truly memorable shots.
Step 1: Choosing an Ultra-Wide Lens
For a Full frame, or FX sensor you only need to go down to 20mm or less. The shorter the focal length, the wider the field of view is.
Ultra-wides are expensive.
The ultra-wide I have is a Tokina 11-16mm. It's hard to come by because its almost always sold out, so I suggest eBay/Carig's list, or waiting, or checking out another lens.
Step 2: Composing Your Shots
Ultra-Wides are not for getting a wide landscape, or taking portraits against a large background.
Ultra-Wides are about getting up close and personal with what you're photographing, be it a person, a flower, or something else.
Step 3: Taking Your Shots
Don't bother trying to get down on the ground and look through the viewfinder, just keep everything on auto. Don't worry if the auto-focus doesn't work (as it doesn't on my D60 and Tokina), some part of the pole will be in focus, and since it's a straight line, it will look nice.
You want to get as close as you can to the subject, often times being almost on top of it.
This can pose a problem with lighting, (lucky the Tokina goes to F2.8), and shadows, so it may take some trial and error (but this is digital, you can take 1000 photos of the same thing).
Step 4: Lines and Angles
Another thing to take into account is angles. By tilting the camera just slightly, you throw everything off, and make for more interesting shots. Take this telephone pole, it's the same one as on the previous step, I just held the camera alittle differently and in a different spot on the pole.
Step 5: Flash and Shadows
These lenses are ultra-wide, they get everything in. So using the built in flash on your camera is a no-no, otherwise you'll get a round shadow on the bottom of the photo.
If you have another flash, you can use that, but you need something to diffuse the flash, or you will get the shadows still. Sometimes bouncing the flash works.
Next you have to watch out for shadows that you cast from the sun, or overhead lighting.
You should be mindful of these shadows before you take a shot, because oftentimes viewing your photos on the small screen on the camera is not enough to see some shadows that may be there.
Depending on where the shadow is, you can crop it out, like in the last 2 photos on this page.
Step 6: Final Tips
When you take portraits, be sure there's enough lighting, because you can't use a standard flash.
Sometimes you can combine all 3: close, lines, and angles, to produce great shots.
Always remember that you are taking the photos, so set it up the way you want, and the way you will like it, photography is an art form, and all art is subjective.