This question came in via Twitter recently:
When you wish to store variables in a long conversation, you should try and use the concept of session variables.
A small tip I would like to add here: make sure that the actual parameter names you use in your different intents are different.
That is, if you were to use a parameter name $geo-city to capture the departure city in the first intent, and you again use $geo-city to capture the destination city in the next intent (which is quite likely as that will be the default choice), then the second value will actually overwrite your first value within the session variable.
So the fix for this is to change the parameter name in the second intent, for example change it to $geo-city-dest. That should do the trick.