The Rebbe's guidance on vows (nedarim) — making them, keeping them, and annulling them:
What a vow is for. Vows serve as personal fences that safeguard one's spiritual life where Torah observance alone may leave room for shortcomings, which is why their guidance is entrusted to the leaders of the generation (IK 1115). A vow that has been made should be fulfilled in full, and even increased — G-d responds measure for measure (IK 3589). A good practice kept regularly, like reviewing a set number of Mishnayos each day, is akin to a vow and should be maintained (IK 1491).
Avoiding vows. Give tzedakah coins and take on good practices without making a vow — say "bli neder" (IK 6565). One who is worried about vows made in the past should avoid making vows in the future and seek annulment for the earlier ones; there is no need for worry (IK 9851).
Annulment (hataras nedarim). When a vow is involved, it should be annulled as a rabbi instructs (IK 10445). On the laws of hataras nedarim for a good custom such as giving maaser, see IK 6873; on the customs of hataras nedarim before Rosh Hashanah, IK 1318.
See also: Halachah and custom – Asking a Rav, stringency and halachic questions; Tzedakah – How much and how to give: maaser, generosity, promptness and joy.