I once came across a novel approach in court records (research during a college history class). Apparently, at that time (very early 1800s), it was a crime in a particular state. A couple was living together out of wedlock and appeared in court on charges. The couple stated their intent to wed, and claimed to be waiting for the circuit preacher to pass through the area, and the judge let them off. Next term of court, same couple, same claim. Memory's hazy, but I think it was the third time they came before the court and made that claim, the judge had them married then and there, with the jury serving as witnesses.
Out of curiosity, I checked a legal code, circa 1860 (it's amazing what you can find online), checked the index, and yep, it was still illegal then. What's interesting is that it specifies that the couple who lives together in a state of fornication may be brought up on charges. That might have been a nod to the difficulty in proving it. The couple could be fined up to $500 (keep in mind this is in 1860) and/or spend 60 days in the county jail, but could suspend prosecution or punishment by marriage (if a marriage could legally take place).
Just something interesting.