The first thing to realise is that any pet you rescue has likely been neglected and possibly abused. This can lead to some short and long term behavior issues. It is the foster's job to help the pet recover. We may have been lucky, but it has taken us a few weeks of extra work and attention to get them on the road to becoming a good pet.
It's also usually your job to get the pet to and from vet visits, adoption events, and meetings with potential adopters. You do not necessarily need to do this yourself, but if not, then you need to coordinate and arrange for someone else to take care of the transport. For us this is the part that causes the most problems. Everything else can be worked in on our time but these appointments are often some one else's.
This will vary by rescue organization but for the most part the expenses, especially the vet bills will be covered by the rescue organization. That said, most rescue organization's funds are limited, so the more you use of those funds, the less that are available to help rescue other dogs. This means if you can provide for most of the needs of the pets you foster (food, bedding, crates, etc) without needing to rely on the rescue, then you will have more opportunities to rescue.
However there are many people who are very generous and appreciate the work that we do and like to be able to do what they can to help. That can range from donations of toys, beds, and food to cash gifts. We see quite a few more of those when we do adoption events than otherwise. We also have a wish list set up for items that we go through a lot, so that people who want to help can. I suspect with some effort and a little bit of Facebook begging we could make it so that our cash output was minimized. But I suspect that would take some time to get there, and then quite a bit of effort to maintain that. It is probably far better to just rescue as you can afford without any subsidy and look at any gifts as a blessing.
One other benefit is that many rescue organizations are registered nonprofits, and any costs associated with your rescues are tax deductible, so save your receipts.
How do rescue animals get assigned?
Most rescues work by pulling animals from kill shelters that are scheduled to be destroyed, or that are asking for rescue help. Occasionally the rescue will contact us to ask if we can help with a specific animal, but mostly our animals come from people contacting us on Facebook, or us seeing Facebook posts of some shelters that we have a great relationship with. Relationships play a huge part in the ability to rescue and to get the animals transported from distant shelters to you.
Puppies are much easier to get adopted out than older dogs. So if you save an older dog, prepare to have them around for a year or more. It is also harder to correct behavior issue with older dogs than puppies. But mostly, once you develop some relationships with shelters you can get heads up on which dogs are the best candidates for rehabilitation. Also, shelters know that there are limited rescue resources, and the last thing they want to do is burn out one or more of those resources by pushing out animals that have little to no hope of ever being a good pet. Most of the pleadings are for dogs that most of what they need is food, shelter, and love.
Other things you should know
The greatest expectation of a rescuer is that when you choose to rescue an animal you are committing to help rehabilitate and provide a home for that animal until it can be placed elsewhere. Not all animals have the same needs. Some will be easier to get ready to adopt than others, and some will get adopted quicker than others. Getting them adopted also means keeping them visible to potential adopters. This means attending adoption events, updating their profile on sites like Petfinder, and marketing them around to other online sites and Facebook groups.
Once you have a potential pet parent it is often your responsibility to make sure that the person adopting is someone who will be able to meet the needs of your foster pet and are going to be the type of pet parent that will be a good match with the pet. Someone who wants a wallflower dog is not a good match for a highly active puppy, and vice versa.
One of the hardest things to realize is there are far more dogs needing rescue than there are rescuers. However, you are not the only rescuer. So if you submit a request to rescue an animal but another rescue gets it, remember that the important thing is that the animal gets to live and be loved. There will be plenty of others and times where your heart breaks as you have to let one go by because you are at your limit.
The second hardest thing is to let go. On average we spend 5-6 weeks getting puppies from the point where they were rescued to being adopted. It can be really hard to let them go. But in the end you just have to remind yourself that this was the goal and that now you can save another life.