For instance, a top heavy plant will fall easily if not secured properly. Wedge the container between the seats or bring along a couple of cinder blocks to steady the plant.
And when driving, take it easy around the corners your not in a race you know. The best time to move any plant is when it's in a dormant state or its growing period has slowed down, usually after it bloomed.
For you armchair gardeners, order your plants from gardening catalogs when the weather is cool. This way, their tender roots won't dry out as easily.