Engine misfire not only causes vibration, but lowers performance and gas mileage as well. It also loads the catalytic converter with unburned fuel, raising its internal temperatures and shortening its life span. Check the spark plug wires, distributor cap and rotor (on cars still equipped with distributors). Look for cracks inside the cap (even a hairline) and burns on the plastic portion of the rotor.  Replace the parts if they aren't perfect.  Inspect plug wires for oil soaking or heat cracks on the insulation, then test each plug wire's condition.

One simple test is to use a low-cost tool that substitutes for a spark plug and provides a rough visual check of the wire's ability to deliver the spark to the plug while the engine is run during cranking or at idle.

Another caveat is to be sure and purchase high-quality replacement wires.  With today's high under hood temps, only the best insulation can cope with the heat. Conventional wiring will simply give up within a few months.

The downside to this new silicone insulation is its mechanical fragility yanking on the plug terminal with a pair of pliers is more than likely to damage the rubber. So you may want to invest in a set of plastic tongs intended to be used for just this purpose. The tongs can be found inexpensively at the auto parts store.

Replace a plug wire if you see external deterioration (particularly at the terminal ends), any looseness of the terminal at each end of the wire can cause a misfire at any time.  Be sure to duplicate the original routing of each plug wire, carefully engaging it to it's plastic guide. Mis-routing of plug wires is a common cause of deterioration and resulting misfire.

The time-honored tradition of pulling a plug wire loose from the plug and holding it next to the manifold while the engine runs is definitely a bad idea.  Aside from the cramped situation in modern engine compartments, the electronically generated spark is substantially higher in voltage than the old-fashioned points and condenser variety. This will get your attention in a big way if the spark happens to find a better path to ground than the gap you've created-say through your elbow to the fender. Worse yet, there's some danger of damaging the expensive ignition  box if you miss and don't let the spark find a home somewhere.