Well for starters, some spells just do not work. If you want to be stronger in your casting, then keep practicing and also work on drawing in energy from the earth. One simple method I have learned is to visualize roots growing out of your crown chakra, down your body, out your feet, and into the soil. You then visualize them sucking up the energy out of the ground.
Another method I have used is to stand straight with your arms at your sides and your feet hip width apart. Tilt your head to the sky and lift your arms, with palms facing up, till they are level with your shoulders. Then you visualize a brilliant white light all around you, filling the area as far as you can see. Visualize this vibrating, brilliant white light streaming into your hands and filling your entire being with pure white energy.
Those are just two of the simple methods, there are tons out there :)