How can I strengthen attic rafters for storage?
Depending on the specific structure of your ceiling, this could be a very bad idea. 

If done badly, it will simply increase the weight on the structure, not add strength at all.

As a general rule, you should be able to put reasonable storage in the roof space without added effort. (boxes of books ok. crates of lead not ok)

If you have concerns though (and you do, because you're asking), you need to speak to a qualified engineer as each roof is different. (Age, Design, Materials, Climate, Prior Use, Condition, supporting structure are just a few important factors)