Otherwise you would be making threads here and there asking why your transaction has not confirmed after 8 or even 24 hours(even more). I recommend a fee of at least(and I mean the very least) of 0.0001 bitcoins, more is better obviously to increase your transaction priority and more likely for it to get included in the next block as fast as possible.
I would recommend a fee of at least 0.0002 per kB. The "per kilobyte" part is because each input and output in your transaction adds to it's size. The fee is evaluated in terms of the data size of your transaction, a 0.0020 fee 20kB transaction would be lower-ranked by miners than a 0.0003 fee 1kB transaction.
In Bitcoin-Qt, the
optional transaction fee that you set is per kB of transaction size. For example, if you send a payment that uses 7 previous payments, it may be 1.7kB in data size, and you would be prompted about the 0.0004 fee that will be included when the optional fee is set to 0.0002.
Besides outspending other transactions to get them confirmed fast,
minimum fee rules of 0.0001 BTC/kB rules also apply for most transactions when they send smaller amounts of bitcoin, to discourage spam. Transactions without this minimum fee will be discarded by the network. While Bitcoin-Qt will always include the minimum fee when it is required and calculate it correctly, other wallets, including a particular web service, don't seem to make this calculation correctly for you; blindly adding 0.0001 may be insufficient if your wallet doesn't tell you how big the transaction will be, you may need to add much more to be safe.