Pack your bag as usual, then take out half of the clothes you originally planned. This way, you won't have to sacrifice style, and you can do some laundry on the road.
Not checking your cell phone plan
It is important to know what your plan covers to avoid data roaming fees, and consider buying an international plan or a local SIM card if data is important to you.
Not booking enough time in between flights
Flight conditions can be unpredictable, so it's best to book flights with a safe buffer in between. Heathrow in London requires a two-hour layover to get from one flight to another.
Not grabbing some local currency at the airport
Take out money from the airport's ATMs for better exchange rates, and keep cash on hand for visiting local markets.