Here is another tip from the online Model Train Club:

Broken couplers are a common problem and this is sometimes caused by hitting the bumpers too hard. One way of avoiding this is to add two small nails just before the bumpers. The nails can be placed exactly two and a half ties from the bumper so they catch the axle of the car and keep the coupler from hitting the bumper. This could save many couplers and lower the frustration level also. You could paint the nails a bright color (eg. yellow) to make them easier to see.