So the last 3-4 times I've ridden, I've worn a bandanna under my helmet. You could really style it any way you want, but this is how I've been using it. A pictorial tutorial, or "PicTuTorial", if you will (please excuse the ridiculous faces I make in photos).
Step one: Place your helmet on your head to get a sense of where the pads come in contact with your forehead. Dorky face and photo is just a bonus.
Step two: Grab your bandanna. Any type will technically do, but I recommend something easily washable, like a basic cotton (as opposed to, say, your pretty silk scarves).
Step Three: To fold the bandanna, lay out out flat. You can just roll it up, but sometimes the triangle ends poke out, which bugs me. It might not bug you. Here's how I fold my bandanna (though, you could just wear it wrapped over your head, I think -- the extra padding from multiple folded layers might discourage forehead-denting better, though).
Step four: put on your bandanna. This summer, I've generally biked with my hair at least part of the way up (it's cooler, since my thick hair is very long right now), and that gives a secure spot to tie the bandanna. From other blogs I've read, and from my running experience, securing the bandanna this way has a triple benefit: protects your forehead from helmet dents, keeps your ears warm, and if you want to cover your ears completely, keeps wind out of your ears. The latter benefits are more applicable to cold-weather riding (or running), but I know some folks who get headaches when they are in the wind with their ears uncovered (including my mama). So, note it if it works for you!
The alternative is to tie it above the bump/hairstyle. This should work with most helmet styles. It definitely works with my Yakkay helmet, as well as my Giro helmet, but it might be to lumpy for a helmet with significant back coverage (Nutcase, Bern, etc). This offers less ear coverage.
Step five: secure your helmet. You can adjust the bandanna so more or less is showing -- this is how it looks with my Yakkay, with is a super-minimal helmet.
And here's how it looks with my more traditional Giro vented helmet.
(As a side note, mini-helmet-review, I prefer the style of the Yakkay, but the Giro feels much more secure and is a little more comfortable from a pure fit perspective. The Yakkay is appealing because it comes with "hat" covers, which I plan to try out in the fall. I find both helmets to be significantly hotter than not wearing a helmet at all [which would be my style and comfort preference - but don't we all feel that way?], but I'm not willing to risk my noggin or its contents to avoid a bump or look chic. At least, not until Americans view bicycling as a legitimate alternative to motorized vehicles. Also, I have the Yakkay Smart One, and the ones currently sold in the US are the Smart Twos. The principal difference is that I have a velcro-close soft strap that wraps around the back and collapses into the helmet when not in use; the Smart Two has a double-metal restraint with a dial-in sizer, which might make the helmet feel more secure. The padding in the Giro is softer, in part because the reason the Yakkay is not as bloody hot as some of the other sleek helmets -- again, see Nutcase, Bern -- is because the rigid pads hold the helmet away from and above your head a bit, allowing air to circulate around your scalp and through the four vent holes in the top. Honestly, it works, and though my head sweats -- it would if I weren't wearing a helmet -- it does not sweat more than in the Giro.)
But I digress...
So, upon reaching your destination, remove your helmet and your handkerchief, and you'll be bump-free! I can't promise no red face, but it's a step in the right direction! Also, unless you've been riding in very hot conditions for a long time, most of the bandanna will be pretty dry, so you can use it to pat dry any super-sweaty spots on your face, chest, or arms. Or to rub at that little bit of chain grease on your calf. Whatever.
When you're not wearing the bandanna, I recommend untying and unfolding it so it can dry. You can probably get two uses out of it if you weren't exceptionally sweaty, didn't have to use it as a towel after your ride, and your back-to-back uses are pretty close together (within a day or two). Whatev. (I should probably note that I have a pretty high tolerance for sweat, non-stinky human smells, and general grime. Maybe it's all those summers in Europe, or at our cabin at the lake in Kansas growing up. I may do a post revealing what a weirdo crunchy hippie I am about most personal care products at some point.)
Finally, after a ride or two, for the love of all that is good and holy, wash the damned bandanna. Head sweat doesn't smell like underarm sweat, but sweat of any kind creates a really hospitable environment for bacteria and it's the bacteria that gets stinky. I used my bandanna for two very short, not-too-hot rides on Thursday and then on Tuesday, and it was fine for both... but when I modeled it for this tutorial, I could tell it was ready for a bath. Et voilà.
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Post script: You may see some changes as I futz with the layout. My primary blog is in Wordpress and so this is a different blogging format for me. For now, I've decided on the photos - you can click on them to see the written post. Cheers and happy riding!