1) Ekadashi as sacred and pious day:
The literal meaning of the word Ekaadasi is one plus ten. The main ten avatars of Lord Vishnu plus as PARABRAHMAM is eleven!
ONE stands for the Lord Vishnu Sashasra Nama Sthothram (Slokam 78, Nama 730) calls HIM "Ekahya Namah" or "Ekasmai Namah" meaning salutations to the ONE. So in the very name of Ekaadasi Shri Hari is present and so it is auspicious day. Since Shri Hari is the presiding deity for Ekaadasi it is called 'Haridina' meaning a day to be dedicated entirely for worshipping HIM, meditating upon Him, listening to HIS glory and studying the scriptures. In short, anything we do on this day should please the Lord.
Mokshada Ekadashi, Margasirsha Shukla Paksha Ekadashi (November – December):
According to prominent Hindu scriptures, on this day, Lord Sri Krishna made the glorious teachings of Bhagavad Gita to Arjuna.
2) Upavas or Fasting:
उपवास, upavasa, masculine gender, meaning fast [ abstinence from all sensual gratification.
Fasting generally means completely abstaining from both food and water (nirjalam). If this is difficult, one may eat a single nongrain meal once in the afternoon or in the evening.
According to scriptures, one who observes fasting is freed from all kinds of reactions to sinful activities and advances in spiritual life. These sacred fasting days greatly help any sincere soul achieve, even within this present lifetime, liberation from the cycle of birth and death. Fasting gives one a real taste of renunciation, thus helping one give up trying to enjoy illusory sense gratification of this material world.
Both western and ayurvedic medicine recommend fasting to maintain and improve health. Indeed modern medial experts and ancient sages agree that fasting benefits one, both physically and mentally. Even from the economic point of view, two such fasting days in a month will save tons of food.
The basic principle is not just to fast, but to increase one’s faith and love for the Divine. The real reason for observing fasting is to minimize the demands of the body and to engage our time in the service of the Divine.
3) Ekadasi Upavas:
"Suddham Bhagavatasyannam Suddham Bhagirathi Jalam
Suddham Visnu Param Cittam Suddham Ekadasi Vratam"
(Hari Bhakti Vilasa 9/283 from Skanda Purana, conversation between Markandeya and Bhagiratha)
'Food from the devotee of the Supreme Personality of Godhead, Ganges water, the mind engaged in the lotus feet of Lord Visnu and fasting on the day of Lord Hari, named Ekadasi, these are all pure and great.'
Ambhareesha Charithram:
The greatness Ekadasi vratham is illustrated by the story of the king Ambarisha. For full charithram see;
http://www.ahobilavalli.org/ekadashi.pdf
4) Rules for keeping Ekadashi Upvaas:
One should not eat any cereals ("Ann" in hindi) like rice, pulses, wheat etc. & salt.If possible, one should stay on water. Drinking luke warm water or lemon-mishri (crystalized sugar) water is very good as it cleanses the hidden undigested food in the body.If one feels hungry, then one can take milk or fruit. Fruit should not be eaten with milk (therefore no strawberry shake, mango shake, chickoo shake etc.). One should not eat food like "sabudana", potato chips, fried food, etc. Also Banana is not recommended on this day since it is heavy to digest.
One should think that he/she is keeping this vrat to please God & to progress further in "sadhana".One should observe self-control.One should do more of maun-japa (repeating the name of God in mind) in the day time.One should do more of Dhyan & Bhajan by keeping awake for a longer time in the night.