Preventing Sluggish Internet

There are two concerns that occasionally haunt me as a netadmin. I’m searching for procedures to resolve these concerns:

(1) When people complain their “Internet is slow” we should be able to answer why and, if it’s in our control, help correct it.

(2) We should know that it’s going to be slow before they do so we can be taking measures to correct it when possible.

Anyone out there with any ideas?