Washing with a sponge and dish soap.
Allowing it to drip dry.
Washing the wheels with too strong wheel cleaner, possibly every time.
Or washing wheels with the same thing they use for the paint.
Using windex or a product like Simply Green on their interior surfaces.
Using the do it yourself engine cleaner to spray under their hood without any planning at all.
Washing in direct sunlight when its hot.
Applying waxes way to think, then driving all over town while it "dries", then wiping it off whenever they get around to it.
In the sun.
Buying whatever goop is on sale to put on their tires. Most times just spraying it on until it covers all the tire without any though about it slinging off and getting on the paint, or the damage some products cause to rubber.
Wrong towels for drying.
Wrong applicators for waxing.
Being led by gimmicky products claiming a year of protection.
Being lazy is a big one.