PB blaster and some stuff called "FREE" thats far from free in cost is what we usually use at work. A few years back I was making my own with a 50/50 mix of diesel fuel and old engine oil/hydraulic oil.
As far as getting the injector out....if its a pencil type injector, and you aren't worried about saving the old one for a reusable core, get your torch out and SLOWLY heat up the body of the injector. Dont concentrate a lot of the heat around where the injector meets the head just in case it heats up too fast. Take a good pair of vise grips and try to get it to rotate a little. If it starts to rotate, DONT GET COCKY! Spray it with whatever lube oil you're using and keep working at it. The more you can get it to move the looser it'll get. BE PATIENT! It might take a few times.
Now, if it comes to the point where you're tired of playing games.....you can use an air hammer....but.....same thing DONT GET COCKY.....turn the settings down or regulate your air pressure down if you have to. Small vibrations will do better than just going at it to beat hell and if you snap it off then you're super FK'd and will more than likely have to pull the head.
I work for a company where the majority of the equipment is subject to de-icing salt so I know a thing or two about how to get stuck parts, frozen pins and broken bolts out. Like I said....BE PATIENT and persistent.