Insect repellent for both you and your horse is a must. An animal can't rest or stand quietly when being eaten by insects. There are many good products on the market. If you use a spray, your horse must be accustomed to its use.
Safe grain bags or 'nose' bags are useful to contain the grain or pellets fed to the horse.
In the spring when your stock is soft, take an extra pack animal. If the animal is young, load light. The same is true if the stock is inexperienced or you are going into especially rough or steep country. Be considerate of your stock and you may have fewer problems.
During the early part of summer your horse will be shed out and short haired. It is common to have frost at night and occasionally snow. If space permits on a pack trip or if you are camping at the trailhead, your horses will rest better and feel better if they are blanketed.