Yes, keep it friendly if you can.
One local real estate was using my image on their home page. I sent them an invoice and they took it down...blaming their web designer for the oversight.
Another was a local graphic designer used another image on an online flyer for an event.
I was a little more abrupt with her as she had previously requested a quote from a direct-contact stock photo agency that I had some images on and then didn't take it any further. Next thing I know, she has used a screen dump of the same image in a flyer.
She paid up when I sent her an invoice.
I have also used a DMCA take-down notice previously with success, but that was a little different - Some muppet had one of my photos on his own Red Bubble site, allowing people to order prints from him and not me. Red Bubble are very quick to act on DMCA notices to their credit.