What I hate is how sunscreen seems to make all the sweat come out of the places it isn't. Also averbenzone stains light colored clothing orange-pink if you're ever so foolish as to try to use a oxygen bleach to get the last evidence of trail mud out of a wicking fabric.
Made myself a helmet cover... on hand stretch fabric was black so it looks kind of dorky, but my ride itself makes that impression inescapable anyway.
Last ride also added a headsweats and seemed to like it, though it wasn't the hottest day.
Just got a long sleeve breathable top I'm going to try, and seriously looking at one of the add-on helmet brims too, though kind of thinking I'll make my own and try to find a way to make it collapsable if I want to take it off towards dusk. Also probably going to do a back-of-neck curtain thing.
There was a while I was considering doing some kind of awning over the helmet vents raised an inch or so that would shade them while still letting air through...