In a previous terminology post on "advice", I pluralized advice to "advices" without even thinking about it.
Alexey Berezkin asks what the plural of "advice" actually should be, since "advices" isn't actually a valid English word. My answer is that "advices" is okay in the context of aspect-oriented programming, since it's basically jargon at that point.
Probably most correct would be "advice classes" or "advice instances", depending on the context. Advice classes are what I write using PostSharp, and advice instances are what get instantiated by the code that PostSharp's post compiler weaves into the rest of the code. "Advices" is sort of an overarching piece of jargon that covers both.