The art of conversational touch was taught to me by numerous gorgeous Ethiopian ladies, who all seem to have this sort of thing down to a science.
Timing is everything. The best way is to wait until you are deep in conversation. When you are deep in a conversation, there is lots of eye contact, talking with your hands, and emoting. Then when you make a strong point about something, grab their elbow while you’re doing it, and hold on to it, lightly, while you are making the point, then let go after you’re done. The idea is to hold onto to their attention so they understand the point you’re trying to make and that their attention isn’t wandering.
In a particularly spirited conversation, these Ethiopian ladies and I would get more physical contact than a lot of my make out sessions. But the key is timing, and reading the situation to see just how comfortable they are with the contact.