Once you've documented the condition of the property before you moved in, you need to maintain it. That means not letting your cat use the dining room as the world's biggest litter box or your kids turn the hallway into a retrospective of their greasy handprints. Also, the carpet in the living room is not an ash tray. Don't put your cigarettes out on it.
Keeping on top of cleaning and minor repairs while you're living in a place will make it easier to get your security deposit back when you move out. Most leases allow for you to get your security deposit back even when things like paint and carpet show "normal wear and tear." Normal wear and tear covers the fading paint or carpet matting. It does not cover the hole your dog chewed in the back door or your adventures with the countertop-staining hair dye in the bathroom.
Keep on top of small repairs as well. If you notice a leak under the sink, for example, get the landlord to fix it ASAP -- otherwise you could be liable for the mold and rotten wood damage that resulted when you didn't make sure the repair got made.
While you may be able to get rid of months or even years of accumulated nastiness when it's time to move out, it's a lot easier to keep the property clean and maintained from the get go, and it makes it more likely that you'll your security deposit back. Plus, staying on top of small repairs and cleaning means you won't be living in your own filth -- and anyone can appreciate that.