I don't know if the game recognizes any specific words, but operating under the assumption that it may, I usually use the words "friend," "love," and "gift" as often as I can. I also don't know if long letters net you better friendship, but I try to fill every line. I almost always attach gifts--and if I'm feeling particularly ambitious, I'll even research the furniture and clothes they like and send them those items.
I don't know if the game recognizes any specific words, but operating under the assumption that it may, I usually use the words "friend," "love," and "gift" as often as I can.
The Japanese game guidebook actually suggests using simple greetings and spacing for easy readability, so I think the game might be able to recognize common words. (Unless this has been lost in translation?) Writing a letter to a villager who has just moved in, who is going to have a birthday, and attaching presents that are things the villager likes will raise relationship points.
I write in simple sentences. I mention "present", "friends" and "thank you" quite often. I always attach a present but I don't have the time to make sure the present always belongs to a theme the villager likes.