Don't give it away.
Hamlet, Scrying Pool, Develop, Lookout, Scheme, Monument, Catacombs, Counting House, Upgrade, Farmland
In
this game, Stef trashes his Potion for a Duchy when there is one Scrying Pool left in the supply. Hamlets are out, Monuments are low and I'm ahead and gaining 5 Monument points a turn. Seeing that Stef cannot pull much money together, I take the Scrying Pool and Province for a 10 point lead with 3 Monuments to end the game.
I hadn't noticed that Stef had been stocking up on Upgrades. These let him empty the Monuments and win by a point.
I'd have been far better off leaving the last Scrying Pool. It's not doing a lot for me, and it gives me more control over when the game ends. If Stef had needed to gain the Potion then he couldn't have also emptied the Monuments for the win.
I can also recommend not ignoring your opponent's buys.