Using the web interface, when you are creating a transaction you can adjust the priority on the transaction.

When creating the transaction, click the More Options link; this will show a Priority field. By default, all transactions are set to send as a High priority. But this can be adjusted to Medium, Low, or Very Low.

Please note that while lowering the priority on a transaction will reduce mining fees, it will also cause transactions to take longer to confirm. Using the Very Low priority setting may cause the transaction to take an extremely long time to confirm; this is something to keep in mind if you are sending a high volume of transactions, or if you need your transactions to confirm quickly.