Contract with your local law enforcement agency to supply an off-duty officer. Law enforcement officers have already been screened at the least for a criminal background and references from past jobs. These officers also have experience dealing with the public and are more apt to keep a calm head in a dangerous situation.
Contract with a security agency that has a trustworthy reputation. Work with the larger agencies, which generally have a larger selection of better-trained officers. Big firms are more likely to fire troublesome and unreliable employees, too.
Place the responsibility of background checks and training on the contracting agency. This makes the security firm accountable if one of its security guards turns out to be an ex-con with a penchant for breaking and entering.
Set the terms of employment at the outset. Establish "post orders" such as no smoking, sleeping, TV and horseplay to outline your expectations from security guards. Require security guards to keep a detailed log of their activities. For example, don't accept: "1:30 a.m., nothing to report," but "1:30 a.m. walked south perimeter to investigate noise; nothing found."
Establish patrol routes that you want covered by the security firm's services. Don't make the area for drive-through patrols too broad, though, or the security guards will drive too fast in order to cover the whole area.
Request the security firm's proof of indemnity insurance and a copy of its license. Ask that you be added to its policy to protect you from any negligence on the part of its security guards. The same applies for any vehicles driven by employees of the security firm.
Read the fine print in the contract carefully to make sure you won't be held liable for any accidents, either those caused by the security guards or suffered by them while on your property.
Require the contracting company to provide its guards with uniforms and anything else they need for the job, including flashlights, notebooks, pens and hand-held radios. Make sure this is written into the contract.