Lead by example. Be compassionate but do not yield to their adultery. Identify yourself as Christian. Be ready to answer questions and be resolute.
The thing about witnessing is, it's communication, and "communication is a two-way street". Sometimes, indeed, you can reach someone by walking into a restaurant and talking to the air. Someone will probably hear you, and maybe these times are when real miracles happen, but we should not pretend we can perform miracles on demand. In order to communicate, you must speak and they must be listening, not just hearing, but listening. Speaking to them as if they're a brick wall is not communicating and thus not purposefully witnessing. They need to somehow be ready for the message.
I hope this helps. I run into the same thing often.