Since there is a huge difference between domestic travel and out of country travel, these pieces of advice should be applied to whichever location fits your situation. Some fit into both categories. At some point, you will probably use ALL the tips, as I have found even veteran travelers tend to not have thought of many of them.
Domestic travel: Print out all your documents and have them handy when you arrive at an airport/hotel etc. There is (now) almost always more than one hotel of the same franchise at every airport and jumping into a cab or on a shuttle to take you to the hotel by the same brand name may not necessarily be for YOUR specific hotel. So make sure you know where you are going and which is the right shuttle for that location. Always ask, ask, ask. Ask the drivers, fellow passengers, desk clerks, maids - anyone who is local to the area that can help you navigate your way through choices of restaurants or malls, safe or unsafe areas and what to see or where to go. Save luggage fees for a second bag by packing lighter and taking a carry-on plus your suitcase instead of two suitcases. May save you $25 right there ($50 for round trip). If you have an e-ticket, use curbside baggage check in to avoid standing in lines to check your one bag. Will cost you a tip and maybe a bag fee but if you are late for a flight, this may save you from missing the flight.
Foreign travel: Make sure you have your needed documents and passports. Some countries require special Visas, some immunization records, some special stamps or documents so ask your airline what is needed when you reserve your ticket. If done online, research online before you leave. If you didn't think of it ahead of time, have the hotel make COPIES of your passports. If you lose a passport, you will help yourself tremendously by having a copy for the authorities. Carry the copy(s) separate from your other documents in case you are robbed. For the same reason, have at least two credit or debit type cards in case one is lost or rejected in a foreign location. Not all ATM's will accept debit cards and neither may your hotel. Keep a list of handy numbers to call in case of trouble. Like bank phone numbers in case you lose your wallet or purse, the local Embassy, the airlines AND your destination hotels in case you get lost. Ask locals for tips on what to see or how best to see it but don't trust strangers to direct you to their 'cousin' who has a jitney or car and "will take you anywhere you want to go cheap".
These simple, common sense tips are often overlooked in the hurry to pack and leave but if you ever had need for any of the tips, you will know why it pays to stop, think and plan ahead - "just in casing".