Your going to have to spend a little money, but here's a few time-proven tips. Lights, absolutely; the more the brighter the better. For some psychological reason unknown to me, it's a proven fact that litter encourages crime. Get other shops in the area to do a thorough clean up. And finally, pole cams, even if they're fakes. Post the area with signs clearly saying that there is video surveillance 24/7, and place the cams where they're clearly visible. If you can afford it, use the real cams and file charges each and every time you review the tapes and find a violation. Good luck. Your's is an uphill battle, but it can be won.
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I train in earnest, to do the things that I pray in earnest, I'll never have to do.