Look for spare keys:
You might have reserved a spare key with the neighbors, your room mate and most probably your landlord. Most landlords keep a spare copy of the keys and might be able to help you if they are nice and are around.
Think like a burglar!
Think hard about the weak points in your house. Imagine all the options a burglar would consider. This may be a left open window, an air conditioner unit or a terrace door. Climb in and get into your place. However, you must take the local guards and the neighbors in confidence before doing that because you might be acting like a burglar but you surely do not want to be treated like one. Another not so great option and particularly for old locks is to deliver a straight downward hammer blow on them. You will of course have to replace it and incur the costs but it is feasible if there are no weak points to get in.
Call the Locksmith:
A locksmith is one person who can help you for sure without damaging any parts of the property. However, if it is a shared service you might have some difficulty in convincing him to open the door for you. If you are able to do it than life gets easy instantly. However you do incur his fee and the time it took.