Get some TIP signs (Turn In Poachers)- they are 12 for $10 at the classic or you can mail the $ to their Pella address (I think it's Pella) and they'll mail em to you.
Also- I spend the EXTRA $ and buy METAL signs... You can get METAL- "Under 24 Photo Surveillance" or video surveillance or whatever- they are about 2 feet tall, thick metal. I can get u a link if needed. They are about $20 each BUT they are awesome, let's folks know you monitor with cameras (which I do), last forever and for around $100- you likely could get your entrances or obvious spots covered by these signs.
The rest of the signs, I try and buy something unique- say a red sign or something that's not just the standard $.99 cent walmart No Trespassing sign