As far as I know, you can't really manually compress your assemblies. However, you absolutely can use IIS compression. From my testing with a bandwidth monitor, it makes a significant difference. And once it's set up, you never have to think about it, it just happens automatically.
I'm surprised this isn't talked about more often. When I wanted to do this a few years ago I could find very little information about it. However, this article should detail all the changes you need to make if you're running IIS 6.0. I'm not sure how much different those instructions will be for later versions of IIS.
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