Sleekeazy’s Hair Potion is a concoction which helps manage and style hair (GF24). The motto is “two drops tames even the most bothersome barnet.” Sleakeazy’s was invented by Fleamont Potter in the mid-1900s and the company he built to manufacture and market the potion was very successful (Pm).
Hermione used "liberal amounts" of Sleekeazy's Hair Potion to fix her otherwise bushy hair for the Yule Ball. It is very effective:
But she didn't look like Hermione at all. She had done something with her hair; it was no longer bushy but sleek and shiny, and twisted up into an elegant knot at the back of her head.
The next day, however, her hair was back to normal because, as she told Harry, "it's way too much bother to do every day" (GF24). She also did her hair up for Bill and Fleur's wedding, since it was also "sleek and shiny," which likely also involved liberal doses of Sleekeazy's (DH8).
