Slick ponytails can make you look bigger.
Sad but true, the fail safe hairstyle for bad hair days can make you appear weightier then you are. Depending on the type of bad hair day, I'd suggest products and a blowdryer to try and save it, Dry shampoo if your hair is too oily to wear out, and a blow dryer to coax wayward hair in the direction you want it.
Big hair looks good almost universally.
I've only recently discovered the teasing comb, and my god, once you get the technique down-pat, it is epically useful. The trick to me is tease the underside, and section by section, going from the front to back, it is much harder to grab a section when it has been partially teased by the previous section. Little rockstar quiffs are fantastic for growing out your fringe, while getting into Audrey Hepburn's Breakfast at Tiffany's look is so much easier then you think after a little practise.
Braids save your ass at the Gym.
The greatest hairstyle for gym is far and away a plait or a braid, much easier to get out of the way and not get it stuck under weights. Because all your hair is one big mass, it's easier to grab it and flip it when it needs to go so you can do your workout properly. Wear it with a soft wide headband if you have a fringe and you're golden.