1. No Leash - Always assume the dog coming to you has no collar, no leash. If this assumption is proven wrong, great! If not, Be prepared and carry some sort of lead (a slip lead, a 2 in 1, or a collar and leash big enough for the average size of your transport). Remember, a standard leash can be converted into a slip lead by putting the metal hook through the wrist strap which should create a loop much like a slip lead.
2. No cage - So, you've got the leash on the dog.... now what? Two options, to cage or not to cage. If you plan on transporting the dog in a cage, you'll need to either have a cage dropped off with the dog, borrow one from another transporter/rescuer, or have your own. Easy way to get one is to go on Craigslist (be careful to avoid scamers and always have meetings with a friend along) and look for a used one in good condition. When in doubt, go bigger. As long as the cage will fit in your car, get a larger cage, because you can put a small dog in a big cage, but just try fitting a Dane in a Doxie cage.
Not going to cage? Make sure you have some plan as to how to secure the dog in the vehicle. Some options include buckleing the wrist strap into the car, using a Carabiner hook to attach the leash to handles, seat belts, or metal hooks in the car, etc. Give enough slack on the leash so the dog can lay down comfortably, but not too much, because the last thing you want is the dog jumping into your lap while you're trying to drive.
3. Unsecured dog! - All it takes is one mistake... ALWAYS have the dog secured. I normally have one lead which secures the dog, and one which walks them so I NEVER have the dog off a lead. Opening the back hatch of a car or a door can be one of the most precarious points in a transport because the dog can slip under/through and is GONE if it is not in some way secured to the car. Think you can catch the dog? Have you seen a herding dog run? Don't risk it, always have the dog secured so even if you open the hatch and walk away, that dog is going NO WHERE. My second transport reminded me of this lesson because the leash I had him secured on was meant for a tiny dog and was cheap (thank you $1 store) and when his 40+ pounds of dog leapt out of the back hatch, he broke the cheap hook like I had tied him up with dental floss. Lucky me, the ping scared him and he paused and stared at me. After watching him play, later on, I realized how BAD that could have been if he had decided to run instead of freeze. Assume your dog is an escape artist, plan accordingly, and you probably wont have a problem.
Also, opening cage doors can be a rough point. Prevent escapes by locking the leash through the cage door (with plenty of slack) so you can have the leash on your wrist before you ever begin to open the door.
4. No Blanket/Sheet - If you are going to be transporting a dog outside of a crate (roaming the car) put down a sheet! Why? Well, crates are an enclosed environment where dogs do not want to eliminate because then they'd have to be close to it and/or stand in it. The open back of your car? Not so enclosed. Thus, put down a sheet (and possibly a tarp under it) to protect your car from possible accidents. This also makes cleaning up fur much easier!
5. Know your Doggy History - if you are transporting dogs who have not been socialized together successfully, or you are unsure if they have been socialized together successfully, put them in crates (not wire cages, not free in the car). Just because they seem to get along when you load them in the car does NOT mean they wont decide to hate each other right after you hit 60mph on the high way and start a fight. Crates keep the dogs from snapping or scratching at each other during the ride. Also, if you discover the dogs want to growl through the bars/vents at each other, think tweety bird and put a sheet/towel up to block their view.
6. "Ponies need food" - Please view the following clip to understand the concept of "ponies need food" That being said, it is always good to know when the dog last ate, was medicated (and when they should receive their next medication if they are on meds), went potty, etc so when they are dropped off you can let their new foster/owner know. Especially important if they are being transported to a vet for a procedure, like being fixed.