A great starting point is the British Thoracic Society's website (www.brit-thoracic.org.uk/guidelines.html). It has loads of in-depth resources, including the famous stepwise guidelines for adults and children.
If you're in a general practice setting, you might find the Asthma UK website useful (www.asthma.org.uk). They've obviously put a lot of effort into making it look appealing, and it has sections for patients and health professionals. I particularly like the animated inhaler demonstrator, which grabs the interest of children that come into the surgery and saves me trying to demonstrate correct inhaler technique. This feature also has animations for the more unusual inhalers, in case you ever come across one you haven't seen before.
Another common problem I have when seeing patients with asthma is that there's never a chart of standard peak flow values when you need it. The website www.peakflow.com includes predicted peak flow graphs for adults and also some standard