This is a broad question, but I'll try to touch on some issues for diabetics.
Diabetics should avoid generally unhealthy activities such as drug use, smoking, excessive drinking, unsafe sex, etc. Since we already have one knock against us, so to speak, healthwise- it's best to eliminate all preventative health concerns.
On a basic level, diabetics should test their blood often, make healthy food choices, and deliver their insulin or other medication in a timely manner. Bolusing for all meals and snacks, if you take insulin, is critical. When people with diabetes don't test their blood, eat high carbohydrate foods regularly, or skip their medication, they begin down a slippery slope that can ultimately lead to devastating complications.
Was there anything specific you were referring to? Please feel free to ask about any behaviors you are concerned about.