Another great slug bait that lasts longer than beer (which goes stale pretty quickly) is mixing 2 Tbsps of flour with 1/2 tsp of yeast in a 500 mg yogurt container, then filling it with water and burying it so the lip is at the same level as the soil.
Personally, I've never found the eggshells or other slug barriers very effective, if your slugs have had a good chance to get their population rocking. The flour/yeast bait combined with hand picking are the only thing that's worked well for me so far. Though I've heard that homeopathy works well too -- that's right, squish up a slug and dilute it a whole lot, then spray it on your whole garden.
I'd be careful with the pest oil sprays, by the way -- just because something is botanically derived doesn't mean it's not toxic or harmful to plants. Neem, for example, will strip the waxy coating off leaves, making the plant far more vulnerable to fungal and viral diseases. Better to create an environment that supports aphid predators like ladybugs and lacewings, if you can.