Being able to effectively communicate with loved ones suffering from dementia can be challenging. Early stages of dementia communication are for the most part likely not dissimilar to communicating with someone not suffering from the illness. As the illness progresses, you will likely need to change not only how you speak but the methods of communication themselves.
The book Mind your Brain, The Essential Australian Guide to Dementia by Kailas Roberts details various strategies that can assist those wanting to better communicate with dementia sufferers.
Some Helpful communications strategies:
Speak slowly and clearly
Make sure the environment is suitable e.g. minimize noise and other potential distractions
Repeat if necessary
Increase volume if necessary
Use a consistent voice
Touch and other exaggerate expressions
Use other methods of communications such as images or items
Be patient with and understanding
You may need to try different approaches and change as the illness progresses.