Sometimes wear gloves. In the cold they keep yer hands warm, in the heat they help with grip when things get a bit sweaty!
Helmets: Always, always, always.
They're not expensive, they're well vented these days, so they don't make you too hot. OK, they may mess up your hair, but think about it, you'll look a lot sillier having to have someone wipe your arse for you the rest of your life!
Off road, if you're not crashing you're not going fast enough, and on road, well, cars. Enough said.
Only my opinion, but I think riders without a helmet look stupid. When I see someone riding without one I get similar feelings of contempt as when I see a pregnant woman smoking.
Rant over.