A few tips:
Pack extra batteries- I normally pack two sets of batteries. One tucked into my battery pouch, which is stashed away in my backpack, and another on me, either in the softshell sleeve or in my pants pocket for easy access, depending on the environment I'll be hiking in.
Carry a squeeze air duster/air blower. If you wear a BTE hearing aid with air PVC tubes, you're going to want this to clean out the tubes. In cold/wet weather, condensation will form in the tubes because of air temp differences in the tubing and your body/ambient temperature.
Comfortable headwear. Get yourself a fleece head cap or something that'll extend enough to cover your BTE hearing aids. If you must improvise, grab a bandana and just make a neck cover out of it with a cap (if you're wearing one). If no cap but you have a bandana, wrap it around your head. This will help drastically minimize the wind fluttering feedback caused by the wind blowing in to the microphone. This, consequently, will drown out important sounds that may be essential, such as others voices.
Carry a compact rugged waterproof case. Anything to safely store your hearing aids in from the outdoor elements when not in use or other reasons such as having backup hearing aids. It also protect the hearing aids from getting destroyed/crushed on accident.
Carry microfiber cloth and cleaning tools. Unexpected situation might arise where cleaning will need to be done.
Get aftermarket hearing aids. If you own those pricey hearing aids prescibed to you by the audiologist, you may want to look in to getting aftermarket hearing aids to wear when going out for that peace of mind. Nothing worse than a 7 grands hearing aid falling in to a river and getting washed away.
Monitor the weather and temperature patterns. Believe it or not, hearing aids and the batteries are sensitive to temperature. In colder temperature, the battery life suffers drastically and the hearing aid performance might also suffer. Be prepared.