The very best thing you can do to improve sharpness is to get a better lens. Since you did not specify which one you use, I am guessing an 18-55mm kit lens which is extremely soft. It improves slightly if you close down the aperture but this is really not much to work with. At 18mm, the maximum is F/3.5 and you can stop-down to F/6.7 to improve things. At 55mm, the maximum is F/5.6 and you must go to F/9.5 or so to get sharper results but you cannot stop down much more or you will hit the diffraction limit.
So there are two things to do:
- Get a sharp lens.
- Shoot at its optimum aperture and focal-length, if it is a zoom.
Before buying a lens it is a good idea to read reviews and see technical analysis of the lens. The best ones I know of are published at Imaging Resource, formerly SLR Gear. They give you a widget (Flash Required) where you can change aperture and focal-length at it gives you a visual representation of the resulting sharpness. Depending on the quality of the lens you will see that some are always soft, some take several stops to sharpen and others can be at or close to maximum sharpness when wide open.