Use the "store" key: "STO->"
Let's say that you had an output of 2.001 and wanted to save it.
Hit the "STO->" button, and then "alpha" (and choose any letter) and hit enter:
Then type in that letter and hit enter! It now shows you the stored value!
You have 2.001
Do Ans -> A
gives 2.001
A
gives 2.001