Try a piece of hardboard under the foam if this is your problem, or perhaps you need new foam cushions. Is the chair you usually sit in at home or work the right size for you? Are your feet able to touch the floor with the seat at knee height, and deep enough to support the length of your thighs, but not so deep as to prevent your back resting on the support?
Try a small pillow in the hollow of your back; it may help in sitting or driving. Adjust the car seat to allow your foot to rest easily on the clutch, and your spine to be supported by the backrest. Let your shoulders drop while driving and keep your neck free and loose - tension can increase back pain.
Have you ever wondered why there is a rail beneath the bar in a pub?
Notice how while standing and putting one leg on the rail, your back stops aching - the intention is that you will have another pint!
Look for a step or a chair strut to put one leg on if you are standing in a shop, in order to stop your back throbbing through the collapse of the back's arch.
If you are ironing, try upturning a box to rest a leg on while standing, or sit on a high stool. Gardening: kneel on all fours on a kneeling pad for weeding, or perhaps sit on the ground. Keep changing positions.
If digging, reduce the weight of soil on the spade and don't twist round with a laden spade.
If you tend to have back pain following gardening, you may be able to prevent the trouble by lying comfortably for 20 minutes, following this strenuous activity. When muscles have worked hard, for you to collapse in a chair is dangerous; the muscles are too tired to support the spine well, but after lying down it is usually possible to resume normal activities. When leaning forward, bend from the hips and knees in a semi-skiing position.
Plan activities carefully to avoid pain; prevention of strain is much more effective than treatment for back pain.
Remember, you only have one back.