Laura,
How terrible.
I wish I had a suggestion for you on how you can ensure this never happens again.
Some ideas:
- Make sure you talk to people who matter and tell them what you are working on. Do it casually, but ensure they understand who is doing the work. Dont hide that you are doing this from the boss.
- Keep somone copied on emails when you send drafts of a report. Do it right from the beginning of the project. Keep doing this through the project, right up to when you send the final version. That makes potential credit-stealing known at least to those who have been copied on the emails all along, and may serve as a disincentive. Can you find casual ways to involve other people so you can copy them on your work?
- Put in your name in the "File Properties" of any XL, Word, Powerpoiint that you author. A lot of people dont realize you can do this. You can access this option from the MS Office Menu. Even if you cant protest now, some day you may have the opportunity to prove that you did the work.
- Type in clearly "Authored By" in the header/footer of documents you prepare and on the cover page too, put in your name. To steal your work, your boss would have to replace your name in two places. Typing in one's name as the author of a document where the real author has not thought to include it explicitly is something people tend to find easier to do than delete a name and type in their own.
A lot of these may not work because I am not aware of your office set up.

I hope you manage to figure out how to prevent it.