The Rebbe's guidance on praying for others, sending a request (pan) to be read at the Ohel, and visiting the resting place of tzaddikim:
Praying for others. Praying for another person is itself a great mitzvah and brings blessing to the one who prays (IK 906, 2873, 7848). Spelling out one's request — even entrusting it to someone else to bring — has real value (IK 2076). When praying for someone's health or wellbeing, mention the person's name with the mother's name (IK 2530).
A pan at the Ohel. A request (pan) read at the Ohel of the Previous Rebbe arouses mercy for everyone connected with it (IK 910), and its power comes from the bond between the chassid and the Rebbe (IK 2290). But a pan does not replace one's own teshuvah and heartfelt prayer (IK 719); mentioning names at the Ohel is most effective together with personal growth in Torah and mitzvos (IK 4569, 4664). Be careful with the wording: don't write harsh or extreme language (IK 1144), and don't ask for strength to accept suffering — ask for the good (IK 1067).
Visiting the Ohel. Prepare properly: pray with a minyan, immerse in a mikveh, give tzedakah and learn Torah (IK 9254); the custom is not to eat (though one may drink) beforehand (IK 589). A visit must be accompanied by more Torah study — especially Chassidus — tzedakah and outreach (IK 7412). A husband's visit to the Ohel brings blessing to his whole family (IK 7395). Special times such as a yahrzeit, Lag BaOmer, Elul and the Ten Days of Teshuvah are especially suited to prayer there (IK 893, 1028, 7016), and marking a milestone such as a bar mitzvah there is valuable (IK 1306).
See also: Prayer – Saying Tehillim.