You're doing who or what where and when?

com·mu·ni·ca·tion

1. The act of communicating; transmission.
2. 1. The exchange of thoughts, messages, or information, as by speech, signals, writing, or behavior.
   2. Interpersonal rapport.
    From Answers.com

 We will, for the moment, ignore the sadly ineffectual communication between nursing staff involving my Dad and concentrate on communication with our beautiful ASD kids.


Whisper.
Curiosity killed the cat, so the old wise chickybabes would have us believe, and every.single.child on this planet is curious.
They want to know what the heck you're saying.
And if you're not playing the let's-scream-louder-and-drown-out-mummy's-Very-Important-Message game...the one where each gets louder and LOUDER and LOUDER until it's a screaming babblefest...then you're breaking the rules!
Whisper.
If they can't hear you over their screeching, they will shut up.
Sometimes, a certain mother (not looking in the mirror) used to mouth her words making no sound whatsoever.
And when their voice is stopped = your message is allowed through.
Worked roughly 8 out of 10 times.

Do not make eye contact.
This is a biggie, particularly in smaller children, where there is this huge impression of goodness knows what lurking in peoples eyes and when they look at a person it's a massive overwhelming feeling of their soul being taken.
Aspie teen still has moments where he asks me to look at the ground; he identifies with Indigenous Peoples who believed that cameras would steal their souls, and that the eyes were the windows to the soul. 
It's a major scarey thing for someone to look directly at ASD kids.
And to insist on them making eye contact...!
Do not make eye contact.
It will also ensure only one sense is being called into action, that of hearing, as using several senses together can collide and not only give mixed signals but it increases anxiety and overwhelms ASD kids with too much info.

Do not change your clothes drastically.
This is more for smaller children, as they get the willies up if their parents' outer skin (clothing) changes drastically overnight (colour/style/texture/laundry powder scent, etc) the kids have to work hard for the better part of the day to figure out if you're who you think you are and not some alien being impersonating their parental unit.
Hence, unexplained hostility, meltdowns, rejection, etc.
Do not change your clothes drastically, especially in the middle of the day or if you can, show your old clothes to your child.
This also covers the huge meltdowns ASD kids go through when you're trying to peel the dirty/sodden/stinking clothing from their wriggling bodies while they're screaming the house down - they think it's supposed to hurt as it's part of their 'skin' (like haircuts).
Plus you're changing the sensation on their skin.
*This also holds for hairstyles, spectacles, facial hair on men, jewellery, etc.


Talk quietly and give plenty of warning.
Break down a period of time and give the children fair warning of when something is going to happen.
IE 'when this show is over' means nothing as kids often have poor concept of what time actually is.
Get an egg timer and an old analogue fashioned clock and tell them "When the big hand is here and the little hand is here, we will get up and do such and such".
Use the egg timer to show them how small periods of time count down, set it for 5 mins and show them on the clock, then show them the passage of time on both egg timer and clock once the alarm goes off.
Talk quietly and give plenty of warning.
Even when the excitement and anxiety begin building over the suggestion of change, stay quiet, calm and serene, remember to whisper to bring the volume back down again, also do not make eye contact in anxious moments, and remind them of the time counting down until something is going to happen.


Break everything down into short, simple sentences/steps.
We are the ultimate computer at one end of the scale while our kids are at the other end, slowly building their mainframe. 
We can absorb huge amounts of info at once, our kids cannot.
Too much overload and, like Windows 3.1, you get them crashing and giving you the Blue Screen of Death (stare).
"Get your blue jumper, put it on and put on your red gumboots" comes out of the processor as "bluered bootjumpers get put put put put" crash.
Break everything down.
"Get your blue jumper"
Wait for the blue jumper to reappear in their hands.
Praise them up.
"Put it on"
Praise them up, again.
"Now, find your red gum boots"
Wait for the gumboots to appear stuck on the ends of their hands and arms, wait til they put them on their tootsies.
Praise them big time.
And THANK the kidlet.
Make a big issue of thanking them for helping you.
Kids will go out of their way to gain praise from their parents and beam with pride when they know they've been helpful.
But break it down into short, simple sentences/steps so they can follow each direction and begin to formulate sequences for themselves.
You have to build the database slowly!