In the case of a steady misfire, isolating the misfiring cylinder is the first step in diagnosing the problem.
Items that can cause a misfire:
Loose/disconnected HT Leads
Check they are all attached properly! (Cured my misfire anyway....)
Moisture in Distributor (Dizzy) Cap
Unscrew, check for moisture, dry off.
Throttle Position Sensor
Head Gasket Failure
A spark plug that shows heavy whitish to brown deposits may indicate a coolant leak either past the head gasket or through a crack in the combustion chamber. This type of problem will only get worse and may soon lead to even greater problems if the leak isn't fixed. Coolant makes a lousy lubricant and can cause ring, cylinder and bearing damage if it gets into a cylinder or the crankcase.
Inlet Manifold Gasket Failure
The Inlet manifold gasket can fail and allow air leaks and coolant/anti-freeze to leak into the cylinder ahead of the injectors. Tell tale signs can include rusty spark plugs on cylinder 1 and 4, sometimes even all four, lumpy idle or misfiring, high emissions, jerky acceleration and deceleration when coming off the accelerator, anti-freeze around the inlet manifold, unburnt fuel and possibly destroyed catalytic convertor.