I just want to start off by saying THANKS!! fostering is the backbone of rescue work and so very much worth all the work! Here is the advice i would give:
1. Make sure you research the rescue, some are not as reputable as others. Mainly this is a gut feeling kind of thing based on the answers to the following questions.
2. Be sure to ask what are your financial responsibilities and what are the rescue's. Our rescue does pay for medical issues but they must be pre-approved by the rescue, also they pay for training classes if needed and collars/leashes. i pay for all food and just happened to have an extra crate
3. Ask what is their policy on preventative meds. one rescue i worked for didnt provide any HW med's and neglected to tell me so that i could decide if i wanted to take that risk.
4. Ask what type of training you are required to provide. (i.e. basic commands or take to a class)
5. ask what type of liability the rescue is willing to accept on behavioral issues. One of my foster's bit my dog the first day, my rescue did pay for all medical expenses on my girl, but some rescue's may not.
I am not trying to discourage you, just make sure you go into this with your eyes open. I found out the hard way about the above issues, just dont want to see anyone else have to go through that.
As far as the emotional issues, well i have to say i get pretty attached to my rescues, but so far every foster i have had i can tell that even though my home is a good home, it is not the best home for them. Every one i could have made work with my family, it is just i knew they would find an even better place. I have to keep telling myself this

just to keep from saving them all. I have noticed that i feel so proud, like a momma sending my kids off to college feeling, when they get adopted out. It is very bittersweet but very possible to handle. With the overwhelming number of homeless animals, there will be a new guy coming into your home shortly after the old guy gets adopted.
Also another bit of advice i would give is to get some reading materials, just about all the books recommended in the book forum are good. Mainly just to help you become a better leader to your "pack". another lesson learned the hard way by me! :rolleyes: When you add this new dynamic to your home, i feel the better prepared you are the easier things will go. So, here are a few i would recommend to people just starting out in fostering.
The Dog Listener by Jan Fennell
The Other End of the Leash by Patricia McConnell
Culture Clash by Jean Donaldson
The Power of Positive Dog Training by Pat Miller
Don't Shoot the Dog by Karen Pryor
Personally, i think every dog owner should have these books in their personal library.

they really helped me learn about how to be a better leader. So much so, i find myself a little more assertive and confident at work now too. :D added benefits!
Good Luck! and THANKS AGAIN!
(p.s. sorry this was so long!)