Let your guests know in the invitation that it's an outdoor wedding, so they can dress appropriately.
Enclose maps in your invitations, so guests can find their way.
Make sure you have adequate bathroom facilities. If the wedding is at home, you might want to consider renting a portable latrine. Many rental places have wedding models, complete with a flush toilet and sink.
Make sure the site has access to electricity.
The general rule of thumb for a sit-down reception is to allow for 15 square feet per guest. Do the math, and make sure your tent is big enough.
If you are having the wedding at a private residence, find out ahead of time where the tent will be placed. That way when you are doing your spring/summer planting, you can choose flowers in your wedding colours and plant them near the tent.
Offer parking that is close by - it's a wedding not a hike (unless you want a hiking wedding).
Let neighbors know you are having an outdoor wedding ahead of time so they won't be outside with lawnmowers and leafblowers.
Make sure your wedding site is in an area that won't get muddy or flood if it rains.
Have mosquito spray handy. Citronella candles are not enough if mosquitoes decide to crash your party.
Planning an out door wedding?