Injections are more to add flavor. Best advice I can give to get juicy BBQ is to cook til tender, but not over cook. Lots of folks don't realize that not only is BBQ tougher if under cooked, it's actually a lot dryer. Food scientists and a lot of guys use words like collagen and gellatin, but neither sounds appetizing to me. I just like fat and connective tissue to render and cook down. Cook too long and it obviously squeezes the moisture out, though. Tender doesn't mean mushy.
Also, don't cook too slow. Cooking faster will help keeping things from drying out, but with certain cuts, especially brisket and back ribs, foiling after bark formation will help. If cooking with water in the pan, use it to your advantage and use the steam to help control bark formation. That means cooking st. louis ribs around 250*, not 225*, and if they're nice and marbled, I don't know that foiling really helps them retain any moisture...at least in my experience. Back ribs are a different story.
One other thing related to over cooking is to not rest your meat too hot. Briskets and butts can rest wrapped in foil for quite a while, but you don't want them actually cooking for hours after they're already nice and tender. I'm guilty of drying out brisket flats, pork butts, and ribs as well from holding too hot or too long. True "resting" is to let the meat come down in temp so that juices are evenly distributed when you slice, but you obviously don't want to let it cool down too much, though. Briskets, for instance, should still be hot enough to be "jiggly" when you slice.
Don't be too hard on your BBQ, though. I don't compare my bbq to what I see on tv. If there are empty plates and pans, I'm happy.
