I grip reallllyyy tight with my legs and pull tighter on my reins give him a swift kick as to say you are a huge brat (thats not the word i use though) right now so cut it out. Beau is so afraid of deer its ridiculous, so this happens somewhat often.
As for trail riding with an english saddle TinyLiny suggested a small backpack, which is perfect. There is that brand called camelback and its a water backpack and has storage in it too and is about the side of half of your back it has the straps of a normal school bag and it clips in the front. they are so light and hold atleast 2 liters of water and snacks etc. i have one and i love it i use it for trails, snowboarding, walks etc.
Your horse is an extension of you.