Here's a quick and dirty rundown on my research
there's a couple ways that are fairly common:
1) broth based
2) juice based
3) brine based
4) off the shelf (FAB, Butcher's, etc)
I tried shooting pork with apple juice mixed with a few things and felt it did nothing for changing the moistness or flavor and it seemed to leave green stains between the muscles. Heat tends to dull flavors, and since pork is typically pulled or sliced, it makes more sense to drizzle fresh, uncooked flavors afterwords.
I tried Chris Lilly's inject recipe and thought it had some potential. It certainly doesn't hurt your product, so its worth trying.
I also tried shooting a brine solution. I felt it improved my moistness, but doesn't do much in terms of flavor. FAB and Butcher's are essentially a brine (with phosphates and thickeners) with a bunch of flavor enhancers (yeast extract, msg). My brine was pretty similar (minus the msg and thickeners) and it seemed to work for improving moistness. I got a trial of Butcher's that I'm going to use on a brisket soon so I'll let you know how it goes.
People who use Butcher's swear by it. Its being used by most of the top performing brisket cookers. They say it makes a big difference. Then again, I know a fellow cook who used it and said he didn't like it and most people he poled preferred his control, un-injected brisket over the Butcher's brisket.
When watching pitmaster's, you'll notice that most of the inject people are using is very thick, unlike anything you could concoct at home. I know Butcher's has xanthan gum in it, and I know Mixon uses Butcher's. Tuffy's inject looked gooey too. What's my point? You don't have to be a rocket scientist to figure out what these guys are using. So if you want to inject like the big boys, unfortunately, you're going to have to get out the credit card.