1. Best time to shoot
Pre-dawn, mid-morning and late evening until after dark, in any season is the premier time to take photographs while in Yosemite National Park.
2. Remember the rule of thirds...
Have your subject occupy one third of your frame to achieve the best composition.
3. What lens to use
You do not need a big lens; 200 MM and below are enough unless you are shooting the dangerous animals. For dangerous wild animals in Yosemite Park, it's recommended you use a larger lens and shoot from the safety of your vehicle.
4. For landscape photos...
Use a 17MM to 200 MM lens and put the subject in the landscape...to capture the sense of place with the animal or subject in its environment.
5. Optimal summer conditions in Yosemite
In June and July, optimal photography conditions in Yosemite would be a blue sky with some clouds in it. If you're out early, when the light comes out, it creates the magenta sky with red and orange clouds.
6. Have patience
If you want to shoot photographs in the outdoors and in a natural wonderland like Yosemite and surrounding regions, patience is a must. It's a waiting game. Accept this and embrace it.
7. Pay attention to the lighting
Contrary to popular belief, do not shoot with the sun directly at your back. Doing so will make the light on the subject flat. Move so the light is at an angle and you'll get more texturing and shadows and a result, a much more interesting photo. Most great photos are either side-lit or have the light coming in from a different angle.