if you dont want to delete it, create another account and forward most of your favorite one to there.
ahmm delete your unimportant messages..or make new folder then you can put your messages there so that its not messy..
I normally do the same. I download all the messages and then delete most of them or at least the ones that are quite old now.
I don't need to save emails in my inbox, I delete a lot. Although, if there are some really important ones I print them off and yes, save them as a txt file.