Humans don't have transformations, so no big power boosts.
Well in timeline order, there's Roshi's
Max Power(and other Roshi techniques like
Thunder Shock Surprise,
Hypnosis/
Sleepy Boy Technique and
Telepathy), Tien's techniques,
Ultra Divine Water,
Mafuba(and other
sealing techniques),
Magic(
Shapeshifting,
Conjuring,
Witchcraft,
Divination, bringing the dead back for a day), Kaioken, Spirit Bomb, Gravity Chamber training, Guru's
Unlock Potential, Yardrat
Spirit Control(which besides being a boost by itself also allows
Instant Transmission,
Cloning,
Gigantification,
Healing and
Forced Spirit Fission), Hyperbolic Time Chamber training, Fusion Dance, Old Kai's
Potential Unleashed(plus a potential second stage of such like
Orange Piccolo and
Gohan Beast),
Selflessness State/No Ego Zone, learning God Ki from Whis, potential God of Destruction
training(or
Ultra Ego if you want to use the manga's version) and finally, if we've got a character with the right mindset, Ultra Instinct.
So yes,
a lot of very useful and powerful techniques, game changing training methods and
none-racial transformations that Earthlings are perfectly capable of making use of. Also, if you want to go beyond just martial arts and mysticism, there's always the option of getting Bulma(and later Dr. Hedo) to turn you into a 17 and 18 style "
Android" for infinite stamina, invisibility to ki sensing, prolonged lifespan*, a decent power boost and only needing to drink water to live.
*Not that technology is the only method of lifespan increase, Roshi, Shen, Tao and Baba are all centuries old. Roshi at least has been known for things like possessing a
Phoenix, drinking the Immortality Elixer/water from the Fountain of Youth(depending on Sub or Dub) and the
Paradise Herb. And then there's exploiting the Dragonballs of course.