Manali is an established and well accessed  base for many treks in Himachal Pradesh, specially for the treks starting from Kullu/Manali/Parvati valley. Even for the treks originating from Lahoul or Spiti are often better organised from Manali.

Manali can be reached either from New Delhi (~ 600 Km, 15 hours ) or Chandigarh (~325 Km, 10 hours) by frequent Volvo/A.C./Push Back/Ordinary buses.

You can fly or by train reach New Delhi as per your convenience from any part within India.

Avoid booking wait listed (WL) train tickets in AC classes (1A/2A/3A) or Chair Car (CC). Book Sleeper class (SL)/Second Sitting class (2S) tickets which have normally more seats/berth and hence much better chance to get confirmed.

From Delhi:

The buses from Delhi to Manali start in the afternoon and evening and reaches Manali in 14/16 hours depending on the bus operator. You reach Manali in the morning between 7 to 9 am.

1) Govt run HRTC (Himachal Road Transport Corporation) and HPTDC (Himachal Pradesh Tourism Development Corporation) buses operate on daily basis.
HRTC buses start from Kashmiri Gate ISBT. From Delhi Airport or Station you can reach ISBT Kashmiri Gate via Delhi Metro service (http://www.delhimetrorail.com/metro-fares.aspx). It is only 4 Km from NDLS Railway Station and can be reached easily by booking an Auto Rickshaw.
To check availability and book online (Login required):
http://www.hrtc.gov.in/hrtctickets
HPTDC buses start from Himachal Bhaban, Janpath (Close to Cannaught Place, CP). To check availability and book online (Login required):
http://hp.gov.in/hptdc/Availability.aspx

2) Several private operators run Volvo/A.C bus services from Delhi to Manali. They have different pickup points spread over Delhi. Choose a boarding point according to your convenience. Check for good reviews and ratings before selecting an operator.
http://www.redbus.in/

From Chandigarh:

You can fly or by train reach Chandigarh directly from some cities within India.

Avoid booking wait listed (WL) train tickets in AC classes (1A/2A/3A) or Chair Car (CC). Book Sleeper class (SL)/Second Sitting class (2S) tickets which have normally more seats/berth and hence much better chance to get confirmed.

Chandigrah to Manali buses start from ISBT Sector 43 (verify before the journey). You can check while taking an Auto Rickshaw/Taxi from Chandigarh Railway Station or Airport. Ordinary buses are available from early morning at 4 am ( HRTC, Haryana Roadways, Punjab Transport etc) and tickets are onboard. For online availability and booking refer the earlier links.

Nearest airport is at Bhuntar (Code :KUU) , in Kullu district. Airport is beside the Manali highway (NH 21) and 50 Km before Manali. Flights are not regular and depends upon weather and season. Check with the flight company website or any flight booking portal.

Returning from Manali to Delhi/Chandigarh:

There are two bus terminus in Manali, one for the state operated buses on the Manali mall and the other one for private buses (5 mins walking). It takes around 15 hours and 10 hours to reach Delhi and Chandigarh respectively.

Regular/Ordinary buses are available from morning to evening. Last being at 9:30 pm in the night. Takes around 15 hours to reach Kashmiri Gate ISBT. For booking check: http://www.hrtc.gov.in/hrtctickets

Volvo bus services are available in the afternoon/early evening between 3 pm to 7 pm. For booking check the above links.

For night accommodation in Manali:

We provide the accommodation either at Vashisht, (3 Km uphill) or at Old Manali ( 2 Km towards Manalsu Nullah) . . However if any trekker is interested to spend more time in Manali, he/she can inquire State operated HPTDC hotels. They provide a wide range of choices of Premium, Deluxe and Budget options in Manali. To check see: http://www.hptdc.nic.in/cir0203.htm#hhami