I have a physical labor job and get occasional lower back pain. What can I do help strengthen my back?

Get some OT. occupational physical therapy may help wearing a velcro back support may help orthotics may help abd crunches to strengthen your abds and take pressure off your back may help.
Core training. You need to focus on exercises which improve your core strength. Exercises that focus on abdominal, lower back, and hamstrings will benefit you the most. Yoga and Pilates are both excellent for improving core strength. Resistance exercise is key. Dead lifts and squats are also effective if done properly. Good luck!
Stretching. you need to strengthen your back by exercising and doing some back stretching exercise try ibuprofen if you are not allergic to it and make sure you take it with meal so it does not upset your stomach not better see a doctor might advise some physical therapy session or see a chiropractor might help with some adjustment.