I work for a pet sitting company, and have done so for the past 10 years or so.
Overall, it's been a wonderful opportunity (a second income for me). Most of the families I work with are wonderful.
You can definitely make good money once things get rolling! The owner of our company has about 8 employees. The ones available all day get some pretty steady work. The key will be to hire good sitters soon...ones you can really count on so you can have some freedom. My boss told me, several years ago, that she had not had a vacation in 12 years! Now, she has hired an office manager, so that takes a load off of her workload!
Advertise everywhere! Make car magnets or signs for car windows. Pass out flyers.
You will run into not pleasant situations. The only bad experience I had was with a guy who had four Greyhounds. His house was filthy, and, while the dog's basic needs were met, they needed more. My boss ended up taking one of the dogs to the vet, with an extensive bill, and was never reimbursed.
That's probably the biggest downfall is having to take care of emergency situations and not knowing if you'll get paid.
Now, in addition to working for this company and going out to take care of animals, I also run my own little business of watching dogs in my own home. This provides us with good Disney money! You can take as many dogs as you want, and turn people away if you are on vacation etc. A lot of people will pay good money to not have to kennel their dogs while they are away. Most of the time I enjoy this job. There are some dogs, however, that I dread getting phone calls about because they are very high maintanance. I have a good amount of "regulars", and I tend to go in spurts with having the extra dogs....so it's not a full time thing for me. I do have a friend, however, who always has a house filled with dogs!!
Let me know if you have any specific questions.