I've had two recent ideas for short stories that I feel would be best written as epistolary pieces where the main character is basically giving a monologue. I know that there will be a lot of 'I's and 'you's, so I was wondering, what can I do to avoid too much of that repetition?
Also, I know "Flowers for Algernon" is a short story composed of journal entries. What other pieces can I read to help me with ideas for writing these?
Any other advice is more than welcome.
Short, simple, straightforward words like "I" and "you" don't really interrupt reading flow with moderate repetition; they're clear and unobtrusive.
The only thing to do right at the start is get some material written. Then you'll be able to deal with problematic repetition, not just by how often the word is repeated, but actually seeing when that repetition is actually problematic, where it actually distracts.
The only other point I'd suggest right now is: what you need to worry about is less repeating the word "I", or "you," which are very common and natural. It's repeating a similar sentence pattern, which is easy to fall into while writing, but can start getting cumbersome. For example:
I went to the store this morning. I wanted some gnocchi. I got to the cash register, but then I realized I had forgotten my wallet.
The problem here is less the repetition of the word "I" - if you used a character name, or third-person "he," this would feel just as odd. So you need to switch up your sentence structure a bit:
I went to the store this morning, to get some gnocchi. It was only at the cash register that I realized I didn't have my wallet.
One handy trick for this is, once you've established the narrator ("I") as the subject, you can shift into describing what he sees and experiences, and enriching associations, rather than just describing his direct actions:
I went to the store this morning. Ralph's has my favorite brand of gnocchi; you can't find it anywhere else. Believe me; I've tried. The cashier was all chirpy and polite right up until the moment I patted my pockets and realized my wallet had stayed home.
This version may take a little more wordcount, but it lets you fill out your text with extra detail (a favorite brand, fussiness, a polite cashier), and a lot more voice (rather than "Person X did Y. Then Person X did Z."). Particularly, notice how giving attention to other things (the store; the gnocchi; the cashier) makes the sentences no longer just about the protagonist, helping solve your difficulties with overusing "I."
