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.
On the board, this one reads: Take that card out of the free cell — then you can move 8 at once instead of 4.