I would simply not tell her, and when the time comes to have the shot just hold her, have her look at you and say to the nurse, "Now." Jab, jab, you're done.
All kids are scared of shots. Some adults are even afraid of shots. It's better if she just learns that shots are a part of life. They do hurt a bit, but they are over very quickly. Some kids go out for ice cream afterwards. That's something to think about.
ETA - Ok, Katie, maybe not ALL kids, but it is very common for kids to be afraid of shots. Totally normal, and I think that's important to say. There is nothing wrong with being scared. It feels wrong, as a mom, because we love our little ones. It's important for us to remind ourselves that we are doing what we believe to e the best thing for our kids and that the pain from a shot is short term but the benefits are long term. It's just important to know that it's ok that they are scared, that we might not be able to do anything to prevent it (even though we will do everything we can) and that it's totally normal.