I have had major back problems since I was 25 I'm now 35... I will say ALWAY try and keep your back straight... this does help.
A few tips....
Prepare your body by warming up and then doing some stretches before you lift and carry.
Keep your feet wide apart
Lower and lift using hips and knees, not back.
Maintain the natural curve of back.
I do stretches every morning and it does help...
I think once you get a bad back you have it for life, you just have to learn to live with it...
I also see a back specialist once a month.... he gives me shock treatment!!!