Wait a while and they will be free again.
If you are sending 1 BTC (that you just received) in a 250 byte transaction, you need to wait 24 hours (1 day).
Send 0.25 BTC, you'll need to wait 4 days.
Send just 0.01 BTC as a 250 byte transaction, and you need to wait 100 days before you'll be allowed to send it for free.
priority is calculated as: #BTC * #confirmations / transaction size.
Very-low-priority is defined as less than 1 * 144 / 250 (1 BTC, 144 confirmations == 24 hours, 250 bytes), and they require a fee.
The tentative plan is for fees to be reduced to
5 mils (0.005 BTC) half a mil (0.0005 BTC) for the next release. Changing the definition of "very low priority" at the same time probably makes sense.
(corrected proposed fee)