Use a Belt to Hold Door Open
If you need to keep a door that shuts automatically open for a while, but don't have a door stopper available, try using a belt to hold the door open for you.
The Door Knob
Wrap the belt around the door knob.
The Other End
Tie the other end of the belt to something nearby, make sure its heavy/sturdy enough to hold the door, you don't want to break anything.
Thats It
This is an extremely basic idea, but hopefully it will help you out someday.