Jason - I know your problem exactly - my husband and I own 2 Great Danes, and when they decide to play - watch out! I'm sure you've seen those big metal rollers that you fill with water, and pull behind your riding mower - but the problem with using them too frequently is that you end up compacting the soil, which makes it harder for the roots to breathe. What you might do instead, is top dress the lawn with compost, which you can lightly rake in, and it will hopefully fill in the low spots, as well as be good for the lawn in general. When our lawn gets bad enough, my husband ropes off a section at a time (like one side of the driveway one year, the other side the next to keep the dogs off) and lightly rototills the area, then reseeds. But that's more like a major renovation. Sometimes dog owners have to make a choice - the dogs or the lawn!