Because there are several different possible causes of those pesky lawn patches, there are also several different ways to solve the problem. The good news is that once you know the cause, you can usually prevent them recurring.

Over-use of Fertilizer or Weed-Killer

One of the most common causes of brown patches is the over-use of fertilizer. Weed killer can also be a problem if it's applied carelessly, but fertilizer is one of the top causes of this issue.

The problem in these cases is that excess nitrogen actually 'burns' delicate blades of grass, first causing them to lose color, and then eventually killing the grass if enough damage was done.

The good thing is that this problem will correct itself within about a month, as excess nitrogen is removed from soil fairly quickly. After a month, try seeding the area, and you should find that new growth will flourish.

In the long term, prevent this problem by making sure you calculate your fertilizer applications correctly (for example, remember that grass under shade needs only around half as much as grass in full sun). Alternatively, cut back on the amount of fertilizer you add and try some alternative methods of adding nitrogen, such as using organic fertilizers or leaving grass clippings on the lawn.

Over-Watering

Too much water can drown grass roots, causing them to rot. The result is dead grass and brown spots. Fixing this problem in the long term requires that you figure out why that spot is receiving too much water.

If you're not over-watering your entire lawn, the problem may relate to the composition of your soil (does that patch have a much higher percentage of sand or clay?), or drainage problems (does the lawn naturally drain to that spot?). Once you've figured out and remedied the problem, you can re-seed that area of the lawn after it dries out.

Animal Visitors

The liquid waste left behind by canine visitors contains plenty of nitrogen, essentially causing the same issue as the over-use of fertilizer. The short-term solution to this problem is also the same. You can hasten the process of nitrogen leaching by watering he area thoroughly (in fact, if you catch the offending animal in the act, you may be able to limit the damage by soaking the grass immediately).

In the long term, the only way to prevent this type of damage is by preventing dogs from visiting your lawn. If it's neighborhood animals that are causing the issue and not your own, check your fence for holes.

Human Traffic

Are your brown patches located in high traffic areas of the lawn? If so, the problem may be that foot traffic has compacted the soil. This causes brown patches because it's more difficult for water to get to the roots of the grass.

Fix this problem by aerating the soil. If it's just a small patch, you can easily do this by plunging a garden fork or other pointed implement into the soil several times. If there several patches, or large patches caused by this issue, you may need to aerate your entire lawn to make sure the problem doesn't spread.

In the longer term, you may want to consider altering the layout of your yard, particularly if your brown patches are more like brown paths. If the reason you have brown patches is that you don't have proper pathways, it might be prudent to add some. An added bonus is that an attractive, well-laid path will make your lawn more appealing.

Disease

Surprisingly, disease is one of the least common causes of brown lawn patches. The most common disease-related cause of brown patches is caused by a fungus that can exist in soil for long periods of time without causing any symptoms. The fungus tends to show itself only when the lawn becomes stressed due to other causes.

The fungus generally appears as small brown spots that grow larger when the weather is warm. The spots usually look like brown rings with an interior of green grass. If you notice these types of spots, have the spot tested to confirm the presence of fungus then apply a fungicide to remove it.

In the long term, you can't completely remove the fungus from your entire lawn. This means that if you know you've got this fungus in your lawn, it's important to keep your lawn healthy to prevent the recurrence of a fungus outbreak.