You can find some very good tutorials on how to debug the most common types of hang, performance, memory and crash scenarios with .NET and ASP.NET here: .NET Debugging Demos - Information and setup instructions. These series are really worth reading, as it's full of information, but it takes quite a time :-)

One common problem for server-side apps is to allocate tons of "big" .NET objects that hit the Large Object Heap. Check out if you do a lot of byte[] or MemoryStream allocations bigger than 85Kb (objects with a contiguous size higher than this will go in the LOH and this heap cannot be compacted) for example. This tipically happens if you do a lot of big file (PDF, images, etc.) generation, copying, etc... This should be done using pure streaming without full size allocations.