1. Know what to expect at the critical connections. Check your flight's carry-on regulations and confirm your flight times. Confirm in advance the timing you'll need to be sure not to miss or run for your transport (be it plane, train or bus).
2. Carry some water and a light snack with you. It makes waiting in lines easier and can get you through a situation where the provided food is unpleasant without sending you into a hungry tailspin.
3. Carry less. Check out One Bag for some great tips. Light packing is so much less of a hassle!