How do I know when to replace my headset? The seal has been broken (plastic piece on a 105 headset) for quite some time. I used to ride in the rain alot but not so much any more. There is a slight bit of friction in the headset but how much do i allow for before I replace it?

Also is this a job I can do myself? Do I need special tools? If so then is it more cost effective to have the bike shop do it?

Thanks.