If you live in an area where you want to put up some no trespassing signs, I would do it sooner than later. It may not help keep everyone off your property, but it certainly can help warn people.
It is also a good idea to make sure your signs are able to be seen. Sometimes areas can get very overgrown and the signs become hidden by trees or brush.
The effects of weather can also fade the lettering depending on the type of sign you have, so it is a good idea to replace them every so often as needed.