Easy way to prevent spam
Filtering - Either a hard content filter at the "mail server end" that does it by header checking only
or software like Mail Washer / Mail Washer Pro. Pro has a few more features but isn't free - the features being the ability to send spam to spamcop, after you've registered with them, let it detect your ISP mailservers and it's good to go can tell it's spoofed.. I don't get any in my gmail account, loads in my ISP based one worst I've see was 100/106 spam some were from "me" as well, which is the annoying part.
ISP is getting good at marking the spam as spam, my address book is all in Mail washer's "friends" list so it's marked as ok even if my ISP get's it wrong, also lets me check to see if they mark e-mail wrong so I don't dump something I shouldn't :D
My Yahoo e-mail - I clear it out often loads of spam in it too.
My Hotmail - a few what I hate is it doesn't tell you if you have new mail if you've got a rule to send it to a folder, then again it is free :P
Spam levels did go down but are back up again seems very variable at the moment...
I'll check my counters and see how bad it really is.