It is possible to run on any terrain barefoot with practice. I advocate starting on a smooth, hard surface such as asphalt or concrete, and then slowly add an ever-increasing variety of progressively more difficult terrain. While it is very tempting to start on very soft, forgiving terrain (such as grass), this greatly limits the feedback your feet receive. The result- poor form may go unnoticed. Ken Bob Saxton (http://therunningbarefoot.com/) describes running on grass as a “dessert”; something that should be enjoyed after running on harder terrain.
In regards to terrain, it is ALWAYS important to watch the ground where you run. The distance you fix your gaze on is dependent on the ruggedness of the terrain. Smooth asphalt with little or no debris will allow you to watch the ground 50 feet in front of you and still be able to avoid obstacles. Very technical trails with lots of rugged rocks, roots, and other such debris will require you to watch only a few feet in front of you.
In either case, you eventually develop foot-eye coordination. Your eyes will scan the terrain in front of you. Your brain will create a cognitive map of that terrain. Your brain will then automatically guide your feet to the areas that are free of debris. This is a skill that can easily be practiced. On smooth, clear asphalt, you can practice this skill by avoiding small cracks, paint marks, or any other “obstacle.” Eventually, you can advance to areas that may have real obstacles. The key is to move at a slow pace in the beginning. Walking on moderately difficult terrain is another excellent training tool.
Another handy skill that develops is the ability to immediately react in the event you step on a sharp object. Your body has a very unique ability to respond by immediately shifting your weight to minimize the damage caused by the object. It is difficult to describe this skill until you experience it. Once it is honed, however, it will allow you to run on very difficult terrain with limited visibility. I have advanced to the point of being able to run on fairly technical leaf-covered trails barefoot. If I do not see an object, my brain has enough trail experience to be able to immediately adjust and shift to prevent injury. This skill will develop as you spend more time on various terrains.
Hills tend to be a hotly-debated topic for barefoot runners. Each individual runner seems to have their own opinions on the best technique for tackling hills. Generally, I recommend using the same form going both up and down hills when running on roads. To reduce the likelihood of injury, it may be helpful to increase your cadence and decrease your stride length.
On trails, I often power-hike up hill. If I do run, I use the same form I use on flat ground. Downhill on trails can be a little more difficult depending on the terrain. If it is non-technical, you can use the same technique as roads. If it is very technical, slow your pace considerably and shorten your stride length.