Mine are simple. I have one that reads "NO solicitors" and under that reads "Beware of DOG." It was printed out on 8"x11" paper in big red letters and laminated. Total cost maybe 10 cents. Easy to replace when it fades.
The other sign is on my garage as you approach the house and it says "My dog can make it to the fence in less than 3 seconds, CAN YOU?" I have never had a problem yet.
We have beware of dog signs at the bottom of the driveway and ADT stickers on windows and doors, even though we don't have ADT. Just makes them rethink their intentions even if our 70lb German Shepherd doesn't deter them. Have No trespassing signs in the wooded areas.