One of the earliest, and best, tricks that I came up with was to have a series of numeric codes to use in text messages.  These codes come across looking like phone numbers and can plausibly be denied, yet my lover and I can have reasonably substantial information exchange with just a few digits.

The codes allow us to signal when we are available to be called (spouse is away) or when the window of opportunity has closed.  They allow us to signal whether there is potential suspicion.

There is even the one code that is to remind us both how much we love each other.  It's used to close almost every sequence.

Bottom line, use numbers to avoid writing and you can safely avoid detection.