A prozbul does not retire [at the end of the shemittah year].
And what is a prozbul?
The lender goes to three Torah observant men that they should be a beis din, and he says to them, “You, the judges, I give over to you every debt I have incumbent on so-and-so and so-and-so, that I may collect them any time that I wish.” And they write a prozbul for him with this text: “With us three sitting as one, so-and-so the lender came and said before us I give over to you etc…'” and the three sign on the bottom either referred to as judges or as witnesses. They can do this also at the end of the year, which is on erev Rosh haShanah before sunset. Some say, that they don’t have to specifically write the prozbul, it’s sufficient for him to say so before them. Even if there is no beis din in his location, he is allowed to say “I hand over my contracts to the beis din which is in such-and-such place.”
There is a law of prozbul missing from this description, wait for 180:16!