Make sure to leave a deposit - not just a reservation.
I reserved my storage unit online for a cross-country move. The facility called to confirm my move-in date the day before, but when I arrived, they said that they did not have the unit I reserved available. They never asked for a deposit when I spoke to them on the phone, but when I got there, they said that they don't hold the unit without a deposit. I ended up having to rent a larger, more expensive unit.