If your Ethereum tx has been submitted, but isn't clearing, then there is likely one of two things happening:

1. Your gas price is too low.
2. Your tx is blocked by an earlier tx.

Case 1: Your gas price is too low.

The first thing you want to do to debug this is look at your tx and see what gas price the tx was submitted at.

In Metamask, you can do this by looking at your transaction history and clicking on the transaction. You will be taken to Etherscan, which will show the gas price.

To resubmit the tx

Checkout http://gitcoin.co/gas/ to see the current network conditions with regard to the tradeoffs between gas price and confirmation times.

In Metamask, you can do this by looking at your transaction history, and clicking 'resubmit transaction' (or speed up transaction). Edit your transaction to give it an acceptable gas price, and click 'submit'.

You just resubmitted the transaction with a higher gas price. Yay!

Case 2: Your tx is blocked by an earlier tx.

To explain this case, we need to explain nonces.

Don't worry, they aren't complicated! They are just an index of how many transactions your account has done. Your first tx is nonce 0, second is nonce 1, third is nonce 2, etc.

If your tx is blocked by an earlier tx, it will not clear no matter how high the gas price. What matters in this case is the gas price of the lowest nonce unconfirmed tx.

To fix this case, scroll you'll need to find the transaction with the lowest nonce and resubmit it.

In Metamask, you can do this by looking at your transaction history, and clicking 'resubmit transaction'. Edit your transaction to give it an acceptable gas price, and click 'submit'.

You just resubmitted the transaction with a higher gas price. Yay!