Many low-rate of speed collision related accidents can be avoided through the use of spotters. It is highly recommended that when backing to use someone to spot for you. This is extremely important when operating the larger 12 and 15 passenger vans where clearance and visibilitiy can be an issue. In the case of the 15 passenger van I strongly recommend the use of a spotter.
Naturally avoiding having to back out of a spot is the safest method, but in those instances having someone to assist you in watching for obstructions and your clearance with other vehicles in tight parking situations will cut down on accidents and minor "fender benders".
When using a spotter be sure that you can see them in your side mirrors. Don't rely solely on your review mirror in backing. Another suggestion is to perform a vehicle walk around prior to starting the engine, this will provide you with a good idea of what is around the van that you must be aware of.
Vehicle safety starts with you the operator.