It is very important you read the tenancy agreement clauses relating to handing in your notice as tenancy agreements can vary with the accepted method in which to do so. All tenancy agreement will state weather you need to put it in writing and post the notice first class delivery, send an email that is acknowledged or orally (very uncommon)
When handing in your notice, you need to ensure you have given the correct amount of time as prescribed in the tenancy agreement. If you are sending the notice by Post, ensure you take into account the time spent in sending the notice as it needs to arrive in time to the landlord / agent with the full correct notice time.
You should always receive confirmation form the landlord / agent that notice has been received and acknowledged.