Most acne medications are applied to the skin. When drugs are applied to the skin, much less gets into your blood than if they are taken by mouth. Drugs that don't get into your blood cannot get into your milk. In the vast majority of cases, the amount of drug that is absorbed into the blood from the skin is negligible, though there are exceptions.
Then there are oral acne medications, such as tetracycline and erythromycin. It is generally thought that tetracycline shouldn't be used while breastfeeding because of its effects on children younger than eight (some say 12), namely staining of developing permanent teeth. However, this risk is very small when nursing. The reason? When you are prescribed tetracycline, the pharmacist will warn you not to take it with milk because the tetracycline binds to calcium and won't be absorbed into your body. Thus if you take tetracycline while nursing, the tiny amount that gets into your milk will bind to the calcium and not get absorbed but will come out in the baby's poop.
Another acne treatment is retinoic acid (tretinoin, also sold as Retin-A), which is usually applied to the skin but can be taken by mouth as well. It is absolutely contraindicated during pregnancy because it has been shown to cause birth defects. However, its use by nursing moms is generally considered safe because so little of the drug is absorbed by the mother's body.