About asking for a LOR and keeping it for a year and then mailing it, you should just mail it as soon as you can (or have them mail it) because they dokeep it in a file for you. Gl!
Be careful. Some schools don't want you to send ANYTHING until the date they specify, and some schools want you to send everything in one package.
To the original poster, depending on the relationship you built with the optometrist, just use your discretion. If you spent many hours with him or her, and its clear they would remember you, it's not too late to ask. If you spent a couple hours shadowing someone three years ago, well then you might want to spend more time with them before asking. Also, it is perfectly okay to ask for a letter, but tell them you won't be needing it for a while, and just keep in touch.
If they want to write the letter while their interactions with you are fresh, that would be their call, but seriously, don't have them give you the sealed envelope a year in advance. Many schools have LOR cover sheets that the recommender needs to fill out and sign (that often aren't available until the applications are available).
In my personal experience, the optometrists that I asked (who ended up writing me a stellar joint letter) were extremely willing to write one for me and were very prompt about it (had them ready within a week). This isn't always the case--I spent a lot of time building a very strong relationship with them, and they are very excited to help me get into school. My point is that you don't just want a letter, you want a GREAT letter. Spend time with them to make sure they won't have any hesitations about you. If you're hesitating about asking, they might hesitate about what to write.
