Never be sure you have enough power or enough power conditioners.
Wind screens.
Planning mic directionality.
Gorillas to help lug the gear.
Knowledge of gating and triggering.
Knowledge of when a high or low pass filter works well.
Lots of water and Gatorade.
Have fun no matter what.

(ps-bring 5 extra mic cables and 5 extra power cords)

Yeah. The basic principles of micing and mixing are (if anything) easier without walls - as long as you have enough power. The problems are the logistics. Lots of gaffer tape. Big binder clips for holding down set lists, notes, music, etc. The meters on your mixer and the readouts on various electronic devices may be impossible to read in direct sunlight - little shades can be constructed with cardboard and gaffer tape. Tarps to cover everything if rain kicks up.

+1

Mixing outside is a pleasure if everything else is kosher. Wind and moisture are the wildcards.

The mixing is relatively easy, but the logistics of outdoor gigs are a potential nightmare.

You probably don't need any special advice on mixing, but I can offer tips from a couple decades of experience doing outside jobs if you want to read on.

Best of luck.

All you need is -

Plenty of PA:
Your PA cabinets and amps will work much harder outside with no walls and no ceiling to contain the sound. A great club PA used outside can sound great up close, might not have the long-throw to push a great distance. [curse you Inverse-Square Law]

Plenty of Electricity (nearby):
Voltage drops significantly over long cables, especially under the high load of an amp-rack or two and stage gear cranked up a little extra.

Suitable shelter:
Rain or shine you will want to have shelter. I don't care what the weatherman says, I always have tarps ready to cover the stacks and racks at a moment's notice. Tarps in the truck/trailer/van don't do any good in the truck/trailer/van. Have them at least partially unfolded and tucked under the back of the stacks - ready to toss up and over in 2 seconds. I've been soaked too many times on days there was supposedly 0% chance of rain. We're always prepared and ready to go 10 minutes after the rain subsides. Be ready for a sideways rain. And don't forget to keep the mics, stagebox, and obviously any electrical distro dry.
Even if it's a perfect sunny day you'll want shade for your mixer & racks. The sun will heat your (usually black) gear to scalding hot very quickly.

[[url=http://[/URL]="http://recording.or…"]And a few more nuggets from another thread on the topic of outdoor preparedness.[/]="http://recording.or…"]And a few more nuggets from another thread on the topic of outdoor preparedness.[/] which would be worth looking over if you're concerned about the logistics.

Outdoor jobs involve a completely different set of problems you need to be prepared for:

FESTIVAL REALITY CHECK
For some reason when people think about putting on an outdoor show, they always picture that idyllic best case scenario. Kicking back in the sunshine, beverage of your choice, listening to some good live music. It's a beautiful image, but not always how these things unfold on the day(s) of the show.

PROMOTERS - promoters and event organizers almost NEVER want to move their event indoors, even if they have a contingency plan to do so. And rain-dates are seldom as successful as the original advertised date, so they will push hard to do the gig outdoors rain or shine. If you're under contract, the show must go on. *If you feel it's unsafe to do so, you have to be prepared to stand your ground and be ready to fight to move it indoors OR be prepared to be sued if you pack it in and go home. If they are being unreasonable, sometimes it's the only smart thing to do. Stage lights, mic stands, electric guitars, and all that wiring - they're all just begging to be struck by lightning. And lightning sucks! - ain't that right Max? It is lethal, unpredictable, and undiscriminating. Even if you're not hit directly, it's another way to lose a lot of your investment in a hurry. Grounded outlets, Power conditioners, surge protectors, ground fault interuptors - are even more essential outdoors.

Thanks for the advice so far. Keep it coming!

Fortunately, we will have a tent covering the performance space and a few rows of seats, so that'll keep the sun and rain off of things, but not the wind, so I'll make sure I have plenty of clothes pins and gaffer's tape. Also, we do have an indoor contingency on-site, so we can decide to setup indoors rather that outdoors right up until ~1hr before start time.

Here's something slightly OT (because it's not sound-related)... What would you recommend for a "stage"? I don't need any height for people to be seen (and we're pretty limited by the height/size of the aforementioned tent any way); I just want to provide a solid floor for the bands to stand on since the tent will be over grass. Any suggestions? I'm going to visit the local equipment rental place today to see what they have. It might suffice to just plop down a couple sheets of 3/4in plywood.

Dropping plywood is better than nothing but not much, the ground is never level and unlevel junctions can be a huge tripping hazard. Setting up directly on the lawn or ground is a very unsafe way to go. Electricity is always "seeking" ground and going directly through a performers body suits it just fine. Been there done that, never again!

jg49, post: 348191 wrote: Dropping plywood is better than nothing but not much, the ground is never level and unlevel junctions can be a huge tripping hazard.

Good point. Hadn't thought of that.

Setting up directly on the lawn or ground is a very unsafe way to go. Electricity is always "seeking" ground and going directly through a performers body suits it just fine. Been there done that, never again!

Also a good point. I hadn't considered the insulative properties of an elevated floor.

Most portable stages sections-at least for band/orchestra-can be used without the legs. They still have interconnects for the platform sections.