In Uttanasana, I like to do a few variations to bring myself deeper in to the pose. Try taking Yogi Toe Lock. Another good one is to bend the knees and bring the palms flat next to your feet. Then work on straightening the legs while keeping the palms flat. Make sure you are bringing weight into the balls of your feet so that your hips stay directly over your ankles.
Complete the Half Sun Salutation by coming back through Raised Arms into Mountain Pose. Do five or more Half Sun Salutations.