Think about what you want to accomplish by going outside the classroom.
Develop a lesson plan that outlines what you will be doing, what students will be doing, and what will be the outcome of the trip.
Select a location that will address your educational goals and will be accessible given time, transportation, and cost constraints.
Dry-run the trip. On your own, visit the site or locations you plan to visit with your class. Go through what you will say and do at each stop. Note the travel times between destinations.
Take care of logistics by calling or visiting ahead of time. Secure money for admission fees, get a fee waiver, or let the students know ahead of time what they will be expected to pay for. Make transportation arrangements as soon as you've decided on a destination.
Do the paperwork in plenty of time to meet the requirements of your institution.
Inform the students, orally and in writing, regarding the details of the trip. Include important information such as where to meet, how to dress, what to bring and what not to bring, how much the trip is going to cost them, what to do if it's raining, and what will be expected of them while on the trip. Have students fill out any forms required by your institution ahead of time so you can maximize the time spent in the field.
Make a checklist of any equipment, gear, or teaching materials you will need to bring on the trip. Look over your checklist after the trip to see what you forgot, or what you might add for next time.
Aim to start on time. Let students know that punctuality is important. Also, do your best to get back on time—students will appreciate it. Develop a backup plan in case something goes wrong.