For out of town races in particular, don't wait until the night before you travel to collect and pack needed items. Rather, make a list of things you wish to take and begin getting them together in the days prior to your departure
Whether you're running in your hometown marathon or traveling out of town, collect everything you need to take to the race site for your workout bag and have it ready the night before the race. Also, pin your race number to the front of your singlet or t-shirt. It's a good idea to take along an extra roll of toilet paper in case there's none remaining when you visit the bathroom or port-o-potty. You'll have enough on your mind race morning, let alone worrying about items you need to wear or take to the starting line.
Plan for all types of weather as conditions can change rapidly. It's better to pack everything you might need rather than having to scurry around a new city looking for clothing and/or accessories at the last minute.
For races held in cold conditions, consider bringing some clothing that you can discard during the race after you warm-up.
If you are traveling out of town, be sure to pack healthy snack foods you may wish to eat the weekend of the marathon. Eliminate the need to search for a grocery store that stocks your favorite foods.
Carry bottled water with you if you will be flying to your marathon destination (if air regulations allow). Flying at high altitudes can contribute to dehydration.
If you're traveling to an out of town race by air, it is extremely important that the running shoes and apparel you plan to wear for the marathon are packed in carry-on luggage so that in the event your baggage is lost or delayed by the airline, you will at least have these "essential" items with you.
Keep in mind that the list below of possible items to pack is not all-inclusive. Factors such as weather conditions, food preferences, etc. are quite variable. Nevertheless, refer to the list, as an aid in helping you organize and pack needed items for the marathon.
Essential Gear - "Carry On" Luggage Items
Other Handy Items
Possible Food Items
If you are looking for some additional help in preparing for and running a marathon whether it be a first time event, aiming for a personal best, or possibly to qualify for Boston:
My Top Recommendation is: The 100 Day Modern Marathon Training Plan