Certain spice makers, such as McCormicks or Lawry's, have excellent poultry spice blends that mix well into the brine or sauce that is injected into a roasting turkey. Generally, what spices or spice blend you use depends on whether you are injecting with brine or a sauce.
When using brine, it is recommended that you only mix savory spices, such as garlic powder, cumin, thyme, rosemary, pepper, onion powder, mustard seed, basil and oregano. This is because brine is mostly water that quickly evaporates off. The water keeps the meat moist and lets the savory flavors absorb into the meat. Injecting brine keeps the meat more tender as it cooks.
When injecting a sauce into the turkey, be aware of where the sauce is going. Unlike water that evaporates, sauces generally get cooked out of the meat. While the flavor of the sauce remains, sauces frequently find their way to the surface. When injecting a turkey with sauce, it is important to include some form of glucose or sugar. Warm honey or maple syrup make excellent additions to the sauce as these aid in creating a sweet and crunchy outer layer, if cooked properly.
When using sauce, use fewer spices than you use with the brine, and leave out the cumin, thyme, rosemary and other spices that have over-powering flavors, except the mustard. Mustard and sugar go together perfectly.