This is been discussed from time to time in the forums. A lot just depends on your own neighborhood. In some places the signs are a deterrent. In other places so many people are known to buy just the sign that it tends to not make any difference.
Amazon carries a whole bunch of different generic signs and decals. I think the most effective ones are probably the ones that say “24 hour video recording in progress.” Because those signs are required by law in some states and I think they are often taken more seriously than just the generic “security” ones.
Previous discussion here: