Just because you have a yard doesn't mean you're ready for a yard sale. Especially if your area experienced a messy rainy season, your yard is probably not ready to have hundreds of people walk all over it without any preparation.
Clean up any toys and manmade objects before you do anything else. Having a lawn full of what might very well be garbage is unsightly and a big turn-off for prospective shoppers -- after all, if you haven't taken care of even your yard, who knows how the items you're selling have fared. Cleaning your yard is a good idea anyway, to make your neighbors more cordial and to prevent them from getting upset when you have a garage sale next door to them.
Clear large natural debris, like fallen branches and large rocks, and do some weeding if you've got an unusual amount of large, ugly weeds. You can use a weed whacker to trim the edges and tidy up the sides of your yard. Rake up large amounts of dead leaves so the green grass can get some air.
Mow the lawn. This should be an obvious one, especially after a long winter of no yard-tending. Mow everything, even little strips that won't be in anyone's way -- it all contributes to the big picture, so you'll want to be sure it's all tidy and attractive. (This is a good chance for your kids or a neighbor kid to earn some money, if you don't want to do it yourself!)
Where there are large patches of mud, sprinkle dry dirt or sand (whichever you won't regret putting into your yard later!) to prevent people from slipping or having to walk through mud piles. Drain large puddles, either by siphoning them off somewhere else or the old-fashioned pail-dipping method, and put caution signs around them if they are still nasty and slippery.
Turn off any sprinklers, even if they are timed to come on at night. The last thing you need is a sprinkler surprising you and your shoppers by turning on without warning! Sprinkler surprises can ruin your items and your customers' experience.
Section off parts of your yard that you don't want trampled, like flower beds or garden areas. People will generally respect even a small string "fence," so if you're worried your yard is going to be un-beautified by the people who will be shopping at your yard sale, take an hour and protect everything with a visible barrier.
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