Spending free cells
A card in a free cell does nothing for you until it comes back out. Worse, it halves what you can carry: each filled cell removes one from the (1 + free cells) part of the limit.
When parking is right
Park a card when doing so uncovers something you need immediately — an ace, or the card that lets a run land somewhere. Parking to postpone a decision is how deals end: four parked cards and no plan is the most common losing position in the game.
Getting them back
Look for a home for a parked card every few moves, not just when you run out of room. The hint on the board calls this out on its own, because emptying a cell is often worth more than the tableau move you were about to make.
How many you really need
Most deals can be finished with two cells or fewer; needing all four is a sign of a hard board rather than bad play. That measure — the fewest cells a deal can be solved with — is the most honest difficulty rating FreeCell has.
A quick test before you park
Ask what the parked card is waiting for. If you can name the card that will let it back out — a black eight for a red seven, or the next card up on its own foundation — park it. If you cannot name one, the cell is likely to stay full for the rest of the game.
On a real board: deal 1
This is deal 1 after 17 hints, exactly as the board draws it. The solid outline marks the card the next hint moves; a dashed outline marks the card it is working towards.
On the board, this one reads: That frees a cell. You can move 4 at once now, instead of 2.
The hint takes the 4♣ out of a free cell and puts it on a column. One cell opens, and the limit goes from 2 to 4 — not up by one but doubled, because the board already has an empty column. Every empty column multiplies what an open cell is worth.
Play deal 1. Open it and press Hint 17 times to reach this spot.