If you want to hit Shoshone, just remember that above about 4000 cfs, its considered a Class IV run due to some significant holes and hydraulics that form at those levels, which are forecast until mid-month. Once it gets below 4K its still a rockin' run. I agree with Eric that that running Grizzley to South Canyon or Newcastle (get as many miles in as possible) to get the feel of the oars on bigger water would be a good idea for a newby heading to the Main Salmon. Just remember that the Main may be about 30,000 cfs...
If the beginning rafter's a kayaker already, then knowing how to read water will be a great advantage. Learning to start your manuvers far in advance, as if one's piloting an ocean liner, will be equally important. Also important will be learning how to use just enough oar strokes to put the boat right where you want to be but not too many that you have to use additional oarstrokes to correct (very inefficient).