CPR on a turtle! Now that's a job and a half don't you think 
Herps are somewhat different when it comes to health care. CPR is generally not an issue for several reasons.
1)Herps get sick very slowly. They get worse slowly. If found on death's doorstep (CPR time), their illness is often so advanced that saving them is futile. Here's my vet plug again, but this is why it's always better to chance taking a healthy herp to the vet than to wait until it's too late.
2)Again, herps are slow. They're heart rates are difficult, if not impossible to read by first-aid'ers, and their respiratory rate vary's so much from species to temperature, to illness, to trauma ect. To be honest, it's hard enough to deterimine death in a reptile let alone when CPR is necessary.
It's awesome that you are interested in helping herps in need. (and other animals of course, but that's another forum
)
If you come across an sick or injured herp here are a few pointers.
1)Bleeding and fractures should be stabalized as in other animals.
2)Mouth to nose respiration can be done (but as discussed above, is there a point?). Keep in mind that you can explode a herps very very delicate and small lungs very easily. Reptiles can utilize anearobic respiration, so I wouldn't have any of 2) high on your to do list.
3)Be familiar with as many species as you can. Sick/injured herps need to be kept at an elevated temperature. This temp should be at the higher half of their prefered optimal temperature zone. This will very greatly from species. The POTZ for a savanna monitor will quickly cook a leaf tailed gecko.
4)Be familiar with the reptile vet's in your area. If your doin' first aid, you need to take them (and the herp owner if it's not you) to the vet.
5)Protect yourself. Many reptiles are dangerous in your own way. Large lizards and snakes can seriously harm you as I"m sure you are aware of. Legal in your area of not (more common in the USA) there are venomous snakes both wild and imported kept as pets. Under any circumstance do not do first-aid on a "hot" herp unless you yourself are a "qualified" keeper.
Good luck, have fun, and I hope you never need to use your skills 
Ian