Are you straining your back by doing lawn work around your house? When the summer weather is nice and you're more likely to cut the grass, you might also start to notice some signs of back pain. As with a lot of common activities, bad posture can turn something that's supposed to be simple into a real problem for your muscles.
One of the best ways to stay in good shape is to stay upright, not hunched. True health doesn't just mean keeping the back straight, though: it requires the whole body. Align the spine, raise your head and make sure you're wearing shoes with proper support as well. Pushing a lawnmower also engages the arms, so keep them straight as well as the back for even tension.
There are also some standard good exercise habits that apply to mowing the lawn. Don't forget to exercise, drink plenty of water and take breaks to keep from overtiring yourself. If you have a lot of lawn to cover, you may want to plan your work so you aren't doing it all at once.
It's worth noting that most of the above applies to the traditional lawnmower. What about a riding mower? While this type might seem more comfortable, don't get too complacent. Sitting improperly can often hurt already stressed backs.
If your back is in really bad shape, you can always question whether you need to mow the lawn at all. Is there some other way to trim the grass, perhaps with shears? There's also no shame in getting someone to help you if you know doing the work itself will be trouble.
In the end, there's no real substitute for a doctor's opinion. Talk to a professional chiropractor for practical, accurate advice.