As someone who planned to have an outdoor ceremony and then had to move it inside when it started raining 30 minutes before we were about to begin my BIGGEST piece of advice for anyone planning an outdoor wedding and/or reception (or any party for that matter) is that you MUST have a back-up plan. For the love of your guests and your own sanity you have to account for mother nature who has a tendency to be a nasty bitch at times. I don't care if you live in San Diego where it's 75 degrees and sunny 360 days out of the year, if your outdoor wedding falls on one of those 5 days where it rains you're shit outta luck.
Now even though rain was what mother nature decided to toss my way you also need to account for either really hot or really cold temps, this pretty much goes for any season you get married in. Yes the months of May, June, Sept. and Oct typically have very pleasant temperatures (which is why they are such popular wedding months) but it's not that unusual for the temp to swing dramatically in either direction during those months and you need to be prepared.
A lot of people will plan ahead and have bottles of water or fans available for those hot summer days or have wraps and space heaters ready for the days that are a bit chilly and those are fine but unless you want all your guests huddling under umbrellas you can't disregard the possibility of rain. I don't know many people that like to sit out in the rain while wearing their Sunday best even if it is under an umbrella.
So please please please ladies, have a Plan B or if you want to be really anal like me even have a Plan C (we ended up having to use Plan C at my wedding). I know you may have dreamed of the perfect outdoor wedding since you were 5 years old but shit happens and the weather is the one thing you cannot control on your wedding day. Believe me you don't want the main memory that your guests have of your wedding to be how uncomfortable they were because of the weather.
For those that have had or are planning outdoor weddings did you have a backup plan?