I can give you a little bit of advice on ergonomics.
Ok, first some anatomy, I'll probably .... up on this because I don't know all the English terms.
But you have your back (the left side here is your back, the right side is your front.)
Ok, so you're sitting, lifting or whatever, all the force of your upper body (+any objects you're lifting) will push on the part where your spine connects to your hip-bones.
Now, between the pieces of bone your spine is made of are pieces those pieces (shown WHITE in the image.)
Now, it's important to always have your back straight! (while sitting, lifting, everything.) (this might sound very much - but just keep an eye out on this, it's important.)
WHY is this important?
Ok, look at the lower part of your back at the 'lumbar curve'. When you DON'T sit up straight, your whole spine will form like a ball, there won't be curves in your spine, just one ball-like line (just, bend your back and you'll know what I mean.)
When this happens, the bone parts of your spine will move, this means that they'll get closer to each other on the inside of the curve.
Now, remember the hard pieces between the bones? (WHITE on the image) .. they will get pushed back, because the bones have gotten closer to each other on the front.
When they're pushed back, they can hit your nerve systems. And that's what hurting you!
Now, when you bend a few times, not much happens, but when you always sit like a potato sack, then these (WHITE) pieces between the bones will shape after there 'new' position, they're forced that way, and they'll get pushed back much easier.
Thus, when you do a lot of sitting like a potato sack and lifting with a rounded back, you'll easier get pain in the lower back.
OK, what are we going to do about this?
Sit straight. When lifting, lift with your legs, not your back.
Now, sitting straight can be hard. Us humans aren't really designed for sitting, but much more for standing and laying down.
So you'll have to set up everything well to prevent and to cure your pains caused by bad ergonomics.
Chair: Sit up straight, position your knees in a 90 degree angle. Your feet should hit the ground, AND your upper leg should lay on the chair at the same point.
Your back should be straight, it's spine should have the normal 'double S-form'. Now, because your legs in comparison to your back is in a 90 degree angle it's hard to keep a straight back because you're legs are getting 'pulled down'.
So you can: sit up straight without touching the back of your chair, this is really tiring though!
Or you should make sure that the back of your chair has an 'anti-kyphosis' (hope I'm using the right term.. )
Just look at a 'good' chair. It should be rounded near your lower back? Why, because if it's like that it support your spine in a way that it can stay in it's original position.
Now there are good chairs and bad chairs with the 'anti-kyphosis' a good chair will 'fit' most people. They're designed in such way that almost 90% of the population can sit on them and have there lumbar curve supported. a bad chair would be one that has a little 'anti-kyphosis' but only works for 1 or 2 people..
Now, you can also get a chair measured up for you. If you really have a lot of pain and have money to spend, you can check into that,
But forcing yourself to sit up straight (and if you never done that that much, it WILL be tiring and it's easy to fall back in your old pattern, but when it does tire you, stand up and walk! Or lay down and rest! Don't be a potato sack or hunchback)
And getting a chair that supports your back in a good way is something that's really important for people that sit a lot (like behind computers..)
Training your back is also something that can be wise when you have a weak back - you need to have those muscles to make you sit up well. Oh, and remember, just like any other muscle and such, don't stress it out to much.. your body should feel relaxed at all times, and if it isn't, adjust something to MAKE it feel relaxed!! All the product you use should be adjusted to YOU, not you adjusting to some product!
Now, if this doesn't help, or it's like a lot of pain, strange pain, pain that doesn't disappear even after much sleeping etc. It could be wise to consult a doctor!
Hope this helps, and if something is not clear (because I'm bad at English terms) please ask! And I'll try to explain something again!