When you really want your kids to hear you when you talk, physically get down to their level.
If you can't squat, pick them up and put them on your lap.
Look them right in the eyes, speak calmly and slowly, and say what you have to say.
Don't be vague or babble and don't try to impress your children with your fabulous expanded vocabulary.
Encourage your children to look you in the eye. Kids can be standing right in front of you and have a bobbly head. They're not paying attention.
You certainly don't need to get down to your children's level to say, "Good morning," or "Are you hungry?" Save this kind of action for serious conversations such as, "Now, Simon, what did you do with Mommy's keys?" or, "Do you understand why you got into trouble for hitting your brother over the head with the bat?"
But don't forget that the most important part of communicating is being a good listener.
Communication can't work both ways when you're doing all the talking and none of the listening.