I like to take bills I'm going to keep and do the following:
I get a very light stream of Luke warm warm water running, to get any surface dirt off. (I don't want foreign matter to get pressed into the bill)
I lightly wipe the bill with my fingers under the water for a few seconds.
Then I dry off any excess water.
Then I take a sheet of clean printer paper, and fold it in half, and put the bill in it.
Then I open up one of my unused hard back books and put the bill, in the folded sheet of paper, into the book.
Then I put two more big heavy books on top of that book, then I put a heavy Ammo can that has 5000 pennies in it, on top of those books.
I leave it there for several weeks.
Then I open it up and take the bills out and put them in sleeves, then in top loaders.
I find this process gets out any superficial bends, and it's flattened out nicely.
It doesn't remove creases, and obviously can't repair slight tears you find in circulated currency, but it's easier to insert them into sleeves and holders.
Just plain water can also get off light pencil marks, and even the yellow counterfeit detection pen markings you sometimes find on your circulation currency.