Experience. If the Digimon has enough experience they can digivolve even by theirselves.
Digivice. If the digidestined has a digivice their Digimon can digivolve.
Crest. If the digidestined has a crest their digimon can digivolve to Ultimate.
A DigiEgg. When a Digidestined has a DigiEgg their Digimon can DigiArmor Energize.
DNA Digivolution. When 2 Digimon have enough power and relationship and hope they can DNA digivolve like Omnimon.
Warp Digivolving. If a digimon has alot of strength or if there is a legend by doing somehting they can wapr digivolve from rookie to Mega, like Agumon and Gabumon.
A Dark Digivolution. If the digidestined is mad or treats his digimon bad will digivolve to a dark Digimon which it can do anything like Agumon and Tai or Guilmon and Takato.
Calumon. With the power of a little friend Calumon the Tamers can digivolve with his powers.
Cards. Tamer have cards which modifies their Digimon and even digivolve them like a blue card.