A better way of manipulating the culling mask. Taken from http://www.gamasutra.com/blogs/CharlesCordingley/20141124/230710/Unity3D_Culling_Mask_Tip.php

CameraExtensions.cs

using UnityEngine;

public static class CameraExtensions {

    public static void LayerCullingShow(this Camera cam, int layerMask) {
        cam.cullingMask |= layerMask;
    }

    public static void LayerCullingShow(this Camera cam, string layer) {
        LayerCullingShow(cam, 1 << LayerMask.NameToLayer(layer));
    }

    public static void LayerCullingHide(this Camera cam, int layerMask) {
        cam.cullingMask &= ~layerMask;
    }

    public static void LayerCullingHide(this Camera cam, string layer) {
        LayerCullingHide(cam, 1 << LayerMask.NameToLayer(layer));
    }

    public static void LayerCullingToggle(this Camera cam, int layerMask) {
        cam.cullingMask ^= layerMask;
    }

    public static void LayerCullingToggle(this Camera cam, string layer) {
        LayerCullingToggle(cam, 1 << LayerMask.NameToLayer(layer));
    }

    public static bool LayerCullingIncludes(this Camera cam, int layerMask) {
        return (cam.cullingMask & layerMask) > 0;
    }

    public static bool LayerCullingIncludes(this Camera cam, string layer) {
        return LayerCullingIncludes(cam, 1 << LayerMask.NameToLayer(layer));
    }

    public static void LayerCullingToggle(this Camera cam, int layerMask, bool isOn) {
        bool included = LayerCullingIncludes(cam, layerMask);
        if (isOn && !included) {
            LayerCullingShow(cam, layerMask);
        } else if (!isOn && included) {
            LayerCullingHide(cam, layerMask);
        }
    }

    public static void LayerCullingToggle(this Camera cam, string layer, bool isOn) {
        LayerCullingToggle(cam, 1 << LayerMask.NameToLayer(layer), isOn);
    }
}