Your pack is part of you during a stage race. It carries all your meager possessions, provides a place to rest your head, keeps you organized and generally is like any other piece of equipment critical for your race success. You may have to try various models to find the one that fits best and that you can run in. Short women particularly have a hard time finding one that fits well. Consider weight and do not go overboard on volume. Capacities in the 20-24 liter range are usually sufficient with 30 liters being the absolute maximum. Anything larger and you will be tempted to fill it up!
Your pack should also have the capacity to be cinched down to reduce its volume towards the end of the race. This will keep the contents from bouncing around as you run. Outside pockets are convenient and should be within easy reach. Some models also have a smaller front pack attached. This is helpful for carrying equipment and food that are needed during the day’s stage. It also balances your kit weight to some extent. Many also sport pockets (or are available as an option) on the shoulder straps to hold water bottles, a very ingenious feature. Some may also have a provision for holding a 1.5 liter water bottle.
Inspect all seams prior to racing and assure they are not failing especially the straps. Preventative reinforcements are a good idea in high stress areas. Small plastic s-clips can be employed to keep zippers closed. If you anticipate attaching pieces of your kit to the outside be sure that there are sufficient and sturdy attachment points. Most are not very waterproof and some type of cover or additional dry bag may be required depending upon race conditions.
Not seeing what you like? For those who are so skilled you can also consider building your own race pack system from various components. Some common pack brands sutiable for stage racing include Raidlight, WAA, OMM, Inov8, Ultimate Direction and Osprey.
Some final insight. How you pack your pack is also important especially early on in a race when your pack is at its heaviest. Try to place heavier items such as food close to your spine and higher up in the pack. This will make it ride better and help with balancing your load. Also make an effort to keep track of things by storing items in the same location in your pack. It becomes annoying to dig through everything to find that one item you are looking for and it reduces anxiety over potentially lost equipment.