Regular use of mouthwash may help prevent or reduce the frequency of sores.
In some cases, switching toothpastes can prevent mouth ulcers from occurring with research looking at the role of sodium dodecyl sulfate (sometimes called sodium lauryl sulfate, or simply SLS), a detergent found in most toothpastes.  Using toothpaste free of this compound has been found in several studies to help reduce the amount, size and recurrence of ulcers.
Dental braces are a common physical trauma that can lead to mouth ulcers and can be treated with wax to reduce abrasion of themucosa.  Avoidance of other types of physical and chemical trauma will prevent some ulcers, but since such trauma is usually accidental, this type of prevention is not usually practical.
Take caution when brushing or flossing teeth, and be extra careful when using a toothpick.
Aphthous ulcer prevention
Oral candidiasis prevention
Deep vein thrombosis prevention
Venous thromboembolism prevention
Chancroid prevention
Drug induced liver injury prevention
Skin cancer primary prevention
Corneal ulcer primary prevention