This includes dust, beverages, food, crumbs, cat hair, and cigarette smoke. Use a small vacuum to clean out air vents, and the keyboard. Open up the bottom of your mouse and check the rubber ball itself, and the inside of the case. You may need tweezers and fingernails to get out all the dust and hair. Don't use alcohol to clean the rubber ball as it hardens the rubber.
Commercial kits can clean mice, also.
Do some vacuuming inside your printer, too. It's tempting to cram every last kilobyte that you can onto your hard disk. You need to give yourself breathing room. When you open a document, the Mac leaves the original where it is and lets you work on a copy. You must have more than the same room additionally, i.e. if your document is 100K , you need 200K free just to type.