The most important thing - especially if you're not using a high-end device like Exergen - is to KEEP IT VERY CLEAN. lol.
People compare their temps to my Exergen frequently. Sometimes, they're way off and there's a reason for that. For example: A couple of days ago, a guy asked me to temp his engine and compare the readings... "Mine is reading 20º cooler, as usual", he told me and kinda sighed in frustration. I took his temp gun and cleaned the sensor with my t-shirt, then handed it back to him. I told him to check it again... now he was 2º cooler than my Exergen, well within spec for measuring nitro engines. The look of amazement/gratitude on his face was priceless.
Keep the sensor area as shiny as a showcar's chrome bumper and your readings will be pretty close.