To fix the issue, pay more fees. 250kB is the limit of how much of a block can be filled with transactions paying only the minimum fee (under the Satoshi client rules). Since this limit is being hit on a regular basis, transactions paying the minimum fee are forced to wait. To overcome this limit, a transaction must pay more than double the minimum fee - how much more depends on how large the block is; miners will not produce large blocks unless correspondingly large fees are paid (because large blocks are more likely to be orphaned, so large fees are demanded to compensate for this increased risk).

Transaction volume has reached the point where transaction fees have a significant effect on confirmation times. I recommend paying at least BTC0.00025 per kB to ensure fast confirmation. If transaction volume continues to increase, even higher fees may be required.