1) Be prepared. Bring Kleenex, even if your allergies are not as bad as mine. Seriously, this summer has just been terrible. Someone in your audience will have bad allergies. That someone will be grateful. That someone is me. A bottle of water is also a lifesaver.
2) Be prepared to improvise (even more than you might need to in the library where you would be able to grab any forgotten/spare supplies or materials). You will probably forget something. One time it was a poor little felt turtle that got left behind where I had been practicing my felt board story. Today I temporarily blew out the portable speaker attached to my iPod that I use for storytime (easier than lugging around and plugging in a CD player). So I sang acapella, with apologies to Jim Gill. This was actually a lot of fun. I pretended I couldn't remember the words and this kids reminded me which verse of "Jump Up, Turn Around" we were on. Next time it happens (oh, it will happen again some time) I will invent some verses too. Maybe hopping on one foot or something?
3)Kids love a stuffed dog. They were happier to see the stuffed dog than me. Oh well. I guess I can't compete.
4) Be flexible. You might need to move if your chosen spot winds up being too close to the road or if the lawn is being mowed. (We did write letters to the city and schools asking permission to use their grounds and asking the maintenance staff to plan lawn-mowing around our schedule, but you never know what is going to be happening on adjacent property owners' lawns!)
5) Have fun! It was great to be outside, reading by the river or on the grass by a playground. It really lent a looser atmosphere than being in the library. Kids could run around and they seemed to be in a better mood than in the winter (the beautiful weather helped mine too!)
6) Read blog posts by other children's librarians for ideas and other helpful tips. Adrienne is particularly great because she posts not only about what she did that week, but how well it went. Look for her Storytime in a Park posts. They are lifesavers.
7) See if you can bring another staff member with you. It is much easier for distractions to wander into view outside than it would be in the library. Even in my library, where there is not a dedicated program room. You will just have to roll with whatever happens to come by. Sometimes it is a dog being walked or a Mustang driving by. One time it was a local bus driver who stopped to use the park's portable errr... facilities. You will be able to keep everyone's attention better if there is another person to share reading duties. And then you can read Elephant and Piggie books! Awesome.
8) Graduation-style, always wear sunscreen! (And bug spray). Seriously, don't forget the bug spray.