Landscape photographers are snobs about shooting times. We only shoot in the morning or evening and we turn up our noses to anyone who shoots during the day. While it is certainly true that morning and evening are the most favorable times for shooting landscapes, I think most landscape photographers don't shoot during the day simply because they don't know how.
Photography during the day tip #1: Include more sky than usual. The problem with mid-day photography is that the light is boring–flat and harsh. I usually find that the only interesting lighting in a landscape is the clouds. I often cut out a lot of the landscape and include lots of sky for great high noon landscape photography.
Daytime photography tips #2: Use a colored filter. If you haven't carefully looked at the light during the day, you may not have noticed how blue it is. That blue cast can destroy an otherwise beautiful landscape. To combat the blue and increase contrast, use a yellow-colored filter. A yellow filter will tame down the blue and produce a more natural landscape. For a more dramatic effect, you can use an orange filter. Even more dramatic yet (best for black and white), use a red filter. You can pick up a yellow filter really cheap on Amazon in 52mm size, or 58mm size, or 72mm size, or 77mm size.
Daytime photography tip #3: No clouds in the sky? Fake it! Many landscape photographers take a few dozen photos of interesting clouds and sun flares so they can add the perfect skies onto landscapes where the skies were less than stellar. Next time you're out and you see interesting clouds, consider taking a few photos for your “in a pinch” folder.
Day time photography tip #4: Shoot a time lapse! Set your DSLR on a tripod and use the intervalometer to shoot one picture every 5 or 6 seconds. Then put the photos in a cheap video editing program and speed up time! You can capture gorgeous short video clips of the clouds quickly moving over the landscape. REALLY fun!
Photography During the Day Tips #5: Make daytime into night. Got a 9-stop neutral density filter? If you do, put it on your lens and watch daytime turn to night. In many situations, you'll even be able to do long exposures by simply using an ND filter. If you have an ND filter but it's not strong enough, put a polarizer on as well. Most polarizers cut 1.5 or 2 stops of light.