Canonbasist909,
The short time tested answer is "a tripod" which provides a stationary and steady platform for the camera but that may or may not be practical for you or answer your question.
More specifically, I'll second everything that Rick said on good technique, practice and reading the manual.
You did not mention under what circumstances or types of shooting produce unsharp results. Since the 70-300 is a tele-zoom lens and has image stabilization (that helps a lot) your shutter speed may be too slow, your subject's moving (and the shutter speed is too slow) or the camera isn't steady or is picking up and focusing on something different than what you want in focus. The telezoom magnifys any unsteadiness and the lenses IS (if on and it should be on pretty much all the time) can only compensate a limited number of stops and your settings and active focus points matter.
In addition to Rick's suggestions, you may want to take advantage your camera's Basic Zone modes that pick the camera settings that best suit what you intend to photograph. Please check page 30 of the manual for descriptions of what those modes are what they best suited for. For instance, I sometimes use Sports mode to shoot wildlife, birds and insects that tend "not to stay still" with decent results.
Hope this help!
LZ
