I second what @mtmail said. You can use Algolia [1] to access older questions/answers. But, if you want to make it more accessible I would recommend to upvote or favorite the question/answer. This way will save your up-voted or favorite questions/answers into your HN account. You can use bookmarking tools for tagging questions/answers. It will help you to organize.
As mentioned on FAQ [2], HN limits the number of showed threads because its ranking algorithm. It depends on thread points and submitted time.
I remember, there is a website which provided weekly top Ask HN threads [3]. This website is useful if you miss threads on previous week.