Does it take thirty seconds and several passes to get to the “point” of point-and-click? Are you cursing your cursor? Do you have to roll your mouse in carefully-timed increments and shake it between websites? It’s probably time to clean it.
If you have an optical mouse, wipe it with a soft (lint-free) cloth dampened with a mild cleaning solution. Clean the pad, too, so that the lines are visible to the sensors.
If you have an analog mouse, it’s slightly more complicated. If it’s wireless, remove the batteries so you don’t accidentally erase your hard drive while you’re cleaning. If it’s not a wireless, be sure not to click the buttons until you’re finished. It won’t hurt anything to wipe it off, too, with the damp mild cleaning-solution cloth, but what you really have to do is get the roller ball out and clean inside it.
Turn your mouse over. You will see a little rubber ball held in by a plastic ring. If you put a thumb on each side of this ring and twist it counter-clockwise, the ring and mouse should fall out. Crawl around on the floor for a while until you have retrieved the ball; it is most likely in the darkest recess under your desk.
The ball is easily cleaned. You can either wipe it with the aforementioned cloth, or press tape to it and then pull the tape off in the manner of someone de-linting a sweater. Set the cleaned ball aside so it won’t roll into mischief.
Look down the mousehole. See those little metal rollers? Clean the gunk off them with a q-tip dipped in rubbing alcohol (you could just scrape it off with a toothpick, or even your fingers, if you want to be gross about it). Then turn the mouse right-side up and gently tap it against the desktop to remove any lingering dust. Put the mouse back in and re-set the circular mousetrap, twisting it clockwise until it locks. Straighten the cord if it’s wired; put fresh batteries in and reset if it’s not. Wipe off the mouse pad before you use the mouse again; after all, that’s where all that gunk came from in the first place.
There now! Isn’t that better?