I have been trying to learn how to do unicycle standstills for the last two weeks, and finally seem to be making some progress. One of the things that I had been practicing was using a stability half ball (Bosu Ball balance trainer, about $100) as shown in the picture below. Luckily, my gym at work has one of these babies. I was practicing core stability by standing on the ball on one foot for up to three minutes. Once you do this, you quickly learn how to move your hands to gain balance. In the beginning, you fall off fairly quickly. One of the things I do is to lift up my arm opposite to the direction of fall, and flex the muscle right under the armpit. More often than not, it is possible to regain balance that way. I also do sit-ups with a 30lbs weight, (3 sets of 20) to strengthen abdominal muscles.
On the unicycle, what seems to help for standstills is to keep your core muscles flexed, and the whole body as rigid as possible. Resist the temptation to make bigger shifts in position by letting your hips do bigger movements – the big movements get you out of balance very quickly. Instead, stay rigid and use the arm technique to try to balance. It is really important to get a feel for where your body and the uni is moving, and to be quick to counteract. I alternate between standing on the pedals and sitting, and make sure to turn around, so that the other arm holds on to the fence or whatever other object gives stability. This balance thing, just like learning to unicycle is a thing where your body does the learning independently of your conscious thought processes. It is like some secret inaccessible part of your brain is getting trained. For me it helps to focus on the ground a few meters ahead of me and to get into a meditative-trance like state while doing this. Such exercise is very relaxing – give it a try.