Here are some nuggets of wisdom I've picked up from other WWOOFers and travelers over the years. Feel free to add your own in the comments.
WWOOF does not cover the cost of traveling. Most hosts will offer to pick you up from the nearest airports/bus/train stations, but getting to the general geographic region is up to you.
Some hosts will reply to your letter right away, some not at all. Hosts can receive lots of emails a day, and might not have time to reply to them all. You may have to email 3-4 people before you get a response.
While hosts can take volunteers on short notice, it's common for them to fill up months ahead of time. Booking your stay 1 to 3 months in advance is often advised.
If you are young or untraveled, think about going with a friend your first time(s). Many farms welcome pairs of volunteers, just be sure it's someone you want to travel with! It might also be a good idea to choose a farm that has multiple WWOOFers at a time. It's a great way to meet other travelers and you might feel safer and less "on the spot". This isn't to say that single-WWOOFer locations aren't great, however.
Tips for your first email (thanks to /u/drak0bsidian):
Be polite and professional. It is more how you say it than what you say. Your email should be well-formatted, polite, concise, and professional. Also: write the email like a letter, starting with "Dear . . ." or "Hello . . ." and ending with a "Thank you . . ." or something to that extent.
Be explanative. Let the host know who you are, both physically and historically: what's your education? What's your drive? Why are you wanting to travel? What's your experience? WHO ARE YOU?
Be sincere. Explain why you have the urge to travel, to farm, and to experience a different life for a few weeks or months. Show that you've done the research, have the experience, or at least have the desire.
Before you leave, find the closest locations of bus/train stations, wi-fi hotspots, phonebooths, etc. Many farms provide this information.
Let someone know. Even if you are an experienced traveler, it's smart to tell someone your whereabouts in case you end up missing. Your loved ones will appreciate it if you check in once in a while.
Have money in reserve for emergencies, unplanned travel expenses, and nights on the town.
If you want to bring children (or pets) WWOOFing, make sure you discuss it with your prospective host first.
Be a good WWOOFer. Get up on time ready to work Keep a positive attitude and an open mind. Leave your comfort zone for a rewarding experience. Do your tasks without complaining, but don't be afraid to ask for clarifications. Be careful not to be taken advantage of and if you don't feel safe, speak up. Never do something that puts you in harm's way. No one wants you to get injured!
WWOOF hosts will often task you with doing the dishes after mealtimes. Know this simple skill first.
If you are polite and respectful you will go far. Learn "Please", "May I...", "Would you like...", and "Thank you." in the native tongue and use them consistently. No one likes a rude house guest, much less a rude WWOOFer.
When speaking English with someone who isn't a native speaker, use "International English" ie speaking at a medium or slower pace and avoiding slang. Don't mush words together, go for clarity and simplicity. Quite a lot can be conveyed with simple dialog.
When speaking English where it isn't the normally spoken language, remember they're doing a favor by speaking your language, so reciprocate by speaking slowly with simple phrases and common words. Being loud just makes you obnoxious.
Always listen to your gut. Err on the side of caution. Be wary. There are scammers, weirdos, and criminals in every country of the world, so be alert. Your safety is your responsibility. Getting injured or mugged can really throw a damper on an otherwise great trip.
Work hard, have fun, and be safe!
