I regularly trail ride a horse that likes to occasionally throw a bunny hop or stop and turn on a dime or jump off to the side when a random butterfly startles him. This resulted in me once being thrown and landing on my tailbone (OWW!)- which was totally my fault, because we were cantering on a trail and all was great and I allowed my self to get too complacent and loose in the saddle. As a result, when something startled him, he spooked by stooping suddenly and whipping around and I went flying over.
The point is that it is a smart idea to ride in a manner that is ready for something to go wrong, especially on a spook prone horse. But the distinction is that it's not a matter of getting loose or "relaxed", it is a matter of learning to ride with attention but not tension. It is a matter of putting yourself in a physical and mental frame of mind that is keyed into the signals your horse is sending you, so you can respond at a moments notice by tightening your legs extra, or sitting more deeply or collecting your reins. By focusing on your horse and not yourself, you should be less likely to tense up because you're not in your head thinking about tensing or not tensing - which as we all know can create a self-fulfilling spiral.
What I've been doing, and it works for me, rather than tightening up in anticiation of something going wrong, is to think of sitting deeply into my saddle, stretching down through my legs and heels into my stirrups (almost like a dressage seat). Even when my reins are slack, I maintain enough contact with the horse so that I can quickly collect my reins. If I feel my arms or shoulders getting tense, I shake them out - just a quick shrug to help reset everything.
I've found that when I'm in such a position, even when my horse throws something funny at me, I'm prepared to deal with it - even more so than if I'd been riding as a ball of tension - because I am solid in my seat and balanced on the horse and also because I can often feel the action coming a split scond before it happens. Plus, it is an enjoyable way to ride (I think for both you and the horse) because it really helps you tune out of yourself and into the horse.
And the best part is...no warbling is required. :)