Simple items that do help...
1- change the oil and filter if they have not been changed recently.. - soudns silly, but this REALLY can make a difference.
2- drive the car for AT LEAST 30 minutes prior to the test. Get there with the engine and exhaust HOT. Avoid situations where the vehicle cools down because you have to wait 15-30 minutes.
3- check spark plugs, wires, remove the boots and look for corrosion.
4- check the cap and rotor to make sure all look good
5- confirm the battery has a good charge. Use a meter and check to see 12.6V at rest and that the voltage is closer to 13.4 or above at the battery when the vehicle is running.
DO NOT assume that because the battery has enough juice to start the car that it is healthy. A battery may turn the car over enough for it to start, yet still be a parasitic draf on the electrical system. A weak battery will drag down voltage, causing the ECU to not get good power, and the ECU can only think straight with the proper voltage. I have seen vehicles pop and backfire while being driven around due to a weak battery.
6- if the vehicle sits for a lot, and runs rough on startup, tap the injectors with a scredriver to make sure that the injectors are firing properly.
hope this helps.