Start by treating the urban planter box as a potentially heavy load, not as a piece of furniture. Wet soil, roots, drainage stones, and water can make even a medium-sized planter much heavier than it looks. If possible, remove the plants, scoop out some soil into buckets, and drain excess water before you move it. Emptying even part of the box can make lifting much safer. If the planter is very large, fixed in place, or too heavy to control comfortably, do not try to prove your strength—use a helper or the right equipment.
Clear a path first and wear shoes with good grip. For a small to medium planter box, use a two-person lift if you can. Stand close to the box with your feet about shoulder-width apart, hinge at your hips, bend your knees, keep your chest up, and brace your core. Grip solid handles or the underside, keep the load close to your body, and push through your feet as you stand. Exhale as you lift, and never twist your torso while holding the box. To turn, move your feet instead.
A smarter approach is often to avoid lifting the whole box. Use a pry bar to raise one corner slightly, then place a block or furniture slider underneath. Repeat on the other corners, and slide the planter instead of carrying it. For longer moves, use a planter dolly or hand truck: tilt the box carefully, slide the dolly underneath, strap it if needed, and roll it across smooth ground. Use a ramp over thresholds, and avoid stairs whenever possible. If stairs are unavoidable, get two strong helpers and use proper lifting straps, or hire someone with moving equipment.
When you set the planter down, bend your knees and hinge at the hips again, lowering it slowly instead of dropping it. Stop immediately if you feel sharp pain, numbness, weakness, or strain. The best way to protect your back is to reduce the weight first, keep the load close, lift with your legs, and use a dolly or sliders whenever the urban planter box is too heavy to move safely by hand.