Writing Your Own Letter of Recommendation: An Offer you Can't Refuse
Every year applicants ask me if it's okay to write their own letters of recommendation when the offer is made by a faculty member. It's not unethical to write your own letter if you're asked to do so. If you'd like to explore the scruples of this issue, take a look at this classic New York Times Ethicist column about the topic.
The mistake I see candidates make in this situation is that applicants don't write very strong letters for themselves, likely because they feel awkward about showcasing their accomplishments fully. If you have the opportunity to write your own letter, craft a glowing one. Be effusive and full-throated in your remarks about yourself! Remember that the letter-writer can choose to tone down the recommendation if s/he doesn't agree. But if s/he does, you've utilized an opportunity fully and bolstered your candidacy tremendously.