work on charging energy. typically done while meditating, but there are many ways to do it such as dance or drumming, visualize energy entering your body.
when you are going to cast a spell, clear your mind of all thoughts not associated with the spell at hand. focus on the one thing.
when you're done casting, ground the energy, you can stomp your feet, or tap the ground or anything else, as long as you feel the energy leaving your body and returning to the earth.
that and practice would help with your spells.