Pendant Wiggle: Shut off the power, and drop the canopy by loosening the fitting that holds it against the ceiling. Tighten the nipple against the mounting strap, then tighten the fasteners holding the strap to the outlet box. Refit the canopy.
Fast Burnout: If a bulb's wattage exceeds the fixture's maximum (see the label on the socket), heat buildup in the shade can shorten the bulb's life. This is a particular problem in pendants with enclosed shades.
Flickering: A creepy crawly could be the culprit. Drawn to light and heat, bugs can get wedged between the bulb base and socket, interfering with electricity flow. Shut off the power, unscrew the bulb, and blow out the socket with a can of compressed air.