Getting dust on the sensor is going to happen sooner or later on every DSLR even if one never changes lenses. If you are aware of the environmental conditions and careful where, when, and how you change lenses, you can minimize the likelihood though not eliminate it.
Most of us keep a Giottos Rocket Blower in our bag since it is effective, simple and quick to use. When a Rocket Blower won't work, a wet cleaning using Sensor Swabs and Eclipse or other sensor cleaning products will be required to clean the sensor. It is not difficult to do it yourself though it is important to use the correct products and follow the instructions to the letter.