I board in my home, and I've learned a lot going along.
1) Always do a M&G - the first couple guests I accepted I skipped this step, and both turned out to be very loud dogs. Very sweet, and they got along well with my dogs, but holy crap. One whine-howl-screamed with excitement whenever you got ready to go outside, the other barked at every sight and sound... all night long. I lived in an apartment building. No good!
2) Secure everything - I have a treat cupboard. My dogs have never once tried to open it. Other dogs are not my dogs. :P
3) Not really a mistake, but a good lesson - there's a huge difference between being a dog lover and loving sharing your home with other people's dogs. You can like every single dog you meet on the street, but it's a big jump from that to enjoying hosting them. Every dog has different quirks, good habits, and bad habits. People tend to mold their dogs, even unconsciously, into behaving in a way that they like and in a way that fits in with their home/lifestyle. That same dog, who is an excellent dog for their own people/environment, may not fit in so well somewhere else. I've watched some very good dogs who just did not fit in that well with my home/lifestyle, which makes the stay harder (not unsuccessful - the dog has fun and is happy, but the stay just requires a lot more effort on my part). I'm getting better at sorting that out, and I've built a good client base of dogs who do fit in really well.