Communicating with someone who is deaf or hard of hearing is actually not a challenge.  What makes communication difficult is an unwillingness or lack of understanding about how to provide ideal circumstances for communication to take place.  Creating ideal circumstances is easy; simply follow these steps:

Be Aware Of Where You're Facing 

Always face the person you are talking to.  Be aware that when writing on a board or covering your mouth, your lips cannot be seen.

Speak Normally, Don't Yell 

Speak normally, don't yell or scream.  Yelling, apart from being embarassing, causes you to speak abnormally, making it difficult for people to understand you.

Don't Speak Quickly

Don't speak too quickly.  What you're saying needs to be processed and figured out.

Make Sure You Are Well Lit (Avoid Backlighting)

Dim areas make it difficult to speechread.  If you have a light source behind you, such as a window, this often makes it difficult to see your lips. 

Reduce Ambient Noise 

When speaking to someone who is hard of hearing, try to reduce surrounding noise.

Explore Ways Of Communicating

Explore alternative ways of communicating, such as writing on a whiteboard, using pen and paper, showing examples of what needs to be done, etc.

Confirm With Clarification

If there's a chance you were misunderstood, a great way to clarify is to have someone repeat what you've said.  This works for hearing people as well.

With practice you'll learn to communicate clearly and effectively.