You can get Himachal roadways bus from ISBT, leaving for Manali in every few hours. Also, you can book a volvo either from RedBus or Goibibo in advance which usually starts from Majnu Ka Tila.
You can book a volvo from kashmiri gate till manali that's a pretty good option.. it will cost you around 1000-1200 one side.....also you can hire a cab from delhi to manali and back it will cost you around 12k.....