How do Arminian Protestants describe salvation if they reject once-saved-always-saved?

You mean Arminian, from Jacobus Arminius, Dutch Reformed like Calvin, but who went to the opposite side of the spectrum. Calvin claimed Augustine as his mentor and made predestination the paradigm through which all theology and God Himself must be understood, treating free will as a mere artifact, a byproduct of the bigger paradigm; indeed Luther in The Bondage of the Will left no room for free will at all, and it was for that belief in absolute predestination that Leo X excommunicated him, not for indulgences. Calvin fudged a little by at least calling free will an artifact, but he still effectively denied it. Arminius reversed this: he made free will the paradigm from which all theological questions must be answered, and predestination the artifact, added only to show that God is not sitting there wondering what will happen, since He knows how men will use their free will. So when both read a predestination passage, such as Acts 2 where Peter says the Jews crucified Christ by the foreordained plan of God and yet did it by their own free will, the Calvinist says the foreordination is the real thing and free will the byproduct, while the Arminian says the free will is real and the foreordination is only added to show God's foreknowledge. The Catholic Church, for its part, has done nothing decisive on this since Thomas Aquinas, who has been accused of leaning predestinarian, though a passage I came across suggests Thomas actually saw predestination and free will working together; and the Church seems to have concluded that this is not something it can solve or dogmatize, and it will go on until the second coming.