I have asthma. What can I do to make sure I don't get an episode while I'm at work?

Take your medication. The most common reason for occurrence of asthma attacks is persistent airway inflammation leaving breathing tubes vulnerable to inhaled irritants & allergens. Regular use of controller medications like inhaled corticosteroids either alone or combined with long-acting bronchodilators or singulair (montelukast) reduces inflammation. An ounce of prevention is worth a pound of cure.
Meds. Be sure to take your prescribed controller meds (ex. Inhaled steroids, long-acting beta-agonists, etc.). Also, be sure to have your prescribed rescue inhaler available to use at the first sign of an exacerbation. If you have known triggers for your asthma, do your best to avoid them in your workplace. Peak flow monitoring at home and work can be a useful tool per your physician's instructions.
Occupational asthma? Many adult asthmatics have what is called occupational asthma. This refers to "triggers" found aqt work which cause attacks. They are otherwise fine in other environments. You may want to check this out if this is the case.