I train my guildies during 5mans. Drop a Lightwell on 5man boss fights and tell them that you will be doing no healing (except on the tank) until Lightwell is gone. They learn fast.
Also something you may want to look into is the RSA addon. It is an addon for announcing your raid cooldowns. We like to announce things like Hymn of Hope / GS / PS / Divine Hymn in the healing channel. I have Lightwell configured to announce in raid when I drop it. I *believe* there is also an option to have RSA announce who has clicked Lightwell. I have this disabled, but it might be useful for you if people are arguing that they did click Lightwell.
More than anything, your dps need to realize that they deal no damage when they're dead. Our dps are usually calling out to each other to use a Lightwell when we are working on heroic raids- they eat it up like candy.