No expository dialogue. Expository dialogue says everything and leaves nothing to the viewer or reader. Avoid this at all costs.
Decide what subtext to get across. Do you want to reveal prior relationships, past experiences? Or do you want subtext in the form of literal subtext, such as code-talking.
Figure out the tone and be sure to note that as an action line before a conversation, or directly underneath the character slug.
Be vague with your dialogue. A line like "Do you remember that night in New York when we made love all night?" leaves no room for interpretation. The only answer is in the form of yes or no, which we want to avoid.
Writing that previous line as simply "Remember New York?" leaves a ton of possibilities. What happened in New York?
Have the other character respond with "I've been trying to forget it" and right there, in two lines, you have just established a prior relationship without ever saying it.
Subtext = vague dialogue + correct tones.