[sassan]

Good advise is already given here.

All I want to emphasis is that there are two main cleaning technique, dry and wet. Dry is useless most of time.

Best Wet cleaning involves use of Sensor Swab (That is specific for every camera model though not a big deal if use smaller swabs on the same size or large sensors) plus use of Eclipse that is the best cleaning solution (Which is nothing but absolute alcohol that evaporates almost instantly, leaving no residue on the sensor) The only exception is 5D for with initial Eclipse solutions were dangerous, causing permanent corrosion, but seems to rectified with the new solutions. Avoid any strong air sprays or vacuums that can cause more harm than good.

Remember you are cleaning the filter (Low pass filter) in front of sensor and not sensor itself but should you leave a permanent mark on it (Believe me not to hard to do, if you are not using the commercial swabs. I know it because I did damaged mine and cost me $300 just to replace the filter, when my 30D was still under warranty), you may have to pay large amounts that is not justified for an old camera.
My most important advise to anyone who is buying a second hand camera is to inspect Sensor first as this is going to become a more and more important problem with DSLRs, as time goes by.

Another advise; Canon is very good in addressing the sensor dirt (Probably they understand that future of DSLRs are dependent to this delicate fact), so if you take your body no mater how old or irrespective of warranter situations, to them, they clean it for free and give it back to you in an hour or so if you are lucky enough to live near a Canon repair center, otherwise, you can send it to them, they do the free cleaning but you may have to pay for the shipping back.

As for mirror, just ignore it. Anything that doesn't get into your picture, doesn't worth any attention.

Best of luck and try not to be hero.