I used to dive with a friend who was asthmatic. I believe the key, as in all things, is to get your doctors approval... and to listen to your body. You know best the things that cause your asthma to flare up. On the advice of her physician, my dive buddy would use an inhaler prior to each dive, helping her lungs remain open.
If it's anxiety that causes your asthma to flare up, make sure you are well rested, and very comfortable diving. You may run into unusual, stess-inducing situations underwater...
I would also suggest being very deliberate about your dive plans. Stick to 60' or less the first set of dives that you do so that you can determine how you feel at that level. If you are okay with that level, and can go deeper, try it in small increments - don't be in a hurry to run with the pack.
And one final note - often when diving w/ a shop (ie., vacation), you will be asked to fill out a medical history form. Many people, right, wrong, or indifferent, tend to just say that they don't have anything wrong with them, and that they are not on any medications. Why? Because many resorts will require you to see a local doctor for approval before diving if they are unsure about the safety risk that your condition poses. Asthma, however, is definitely one you don't want to gloss over - in the event of an attack underwater or on the boat, you want the DM and captain to know what's wrong, and how to help you. So, my recommendation is that you obtain the generic medical release form from your certifying agency (PADI has a release form on their website), have it filled out by your physician and take it with you. Some might think that's overly conservative, but I'd rather be safe than sorry, and I'd rather not be evaluated by a local physician when I'm away from home.
Good luck!
