Spread the words. Email, cards, facebook invitations etc. Three things must include: when to eat, what to eat and rain plans, in case it rains, you need to be prepared (e.g. a gazebo) or back up places ready.
Renting items can be cheap. Renting gazebos, foldable chairs, long tables, some vendors are willing to drop off and pick up.
Create the atmosphere by using lighting. You need a little ambient lighting, get those solar LED String lights to hang on the wall, tree branches or the fence. If you have powered lights make sure you do have a extension cord just in case it’s far away from your powerpoint.
Decoration with colours. Go to the $2 dollar store, buy things people can wear, e.g. sunglass, necklaces, it will be fun for photographs.
Away from insects. Try not to select your party spot close to water/ stream but if you have to, provide guests with bug spray or insect repellant.
Plan your music well. No matter classic, jazz or rock. You know what's your party about, and prepare at least 5 hours worth of music. Extra tip, get a good speaker!
Entertain the kids. Ideas like blowing bubbles, slip and slide, video games, rugby balls etc, anything to keep them busy and let them serve themselves.
Get a shed. A marquee or pop up gazebo can always create some nice indoor atmosphere. Most gazebos have sidewalls, so you can create some pretty cool indoor spaces. Make sure you peg down/secure your gazebos in case of high wind or rain.