The Rebbe's guidance on preparing for prayer and on the chassidic way of davening with contemplation (IK = Igros Kodesh letter number):
Contemplate before you pray. Before each prayer, think about G-d's greatness — at least before each of the three daily prayers (IK 3705). Daily contemplation before prayer brings great benefit, spiritually and practically (IK 4731, 6654). The contemplation can be divided over several days (IK 6628), and it doesn't have to follow the exact words of the discourse — understanding it in your own words is what counts (IK 3762). Real preparation includes humility and submission before G-d (IK 403). On devotion in prayer and the prayer of the poor that comes first, see IK 6000, 6054; on prayer and Torah and transforming the worldly, IK 7547; on the deeper meaning of prayer, IK 3423.
What to do before davening. Learn Chassidus before praying (IK 3021, 3550); Tanya is especially valuable then (IK 8299). Give tzedakah beforehand (IK 6654, 6628). Immersing in a mikveh before prayer is a valuable custom (IK 8546). Don't drop the customary preparations before prayer, even out of zeal to perform a mitzvah on time (IK 5751). Thinking over Chassidus between Yishtabach and Yotzer is permitted and prepares one for what follows (IK 2341). Proper conduct and dress in prayer (IK 3055).
Eating before davening. Where health or focus requires it, eating before prayer — such as mezonos or other morning nourishment — is appropriate; the Rebbe cited the Baal Shem Tov and the Tzemach Tzedek on this (IK 1356, 1660, 2178, 3284).
Davening at length. Lengthy, heartfelt prayer is valued (IK 4094). Keep it up among young people too, as it strengthens them against outside influences (IK 1520, 1278). On Shabbos especially, pray at length rather than rushing home after the minyan (IK 3315). Deep intention in one's own prayer comes before hurrying to hear Kedushah (IK 3567). One who prays slowly or on his own can still unite with the congregation in the deepest sense (IK 1196).
But within limits. Don't postpone the start of Shabbos morning prayers until midday; begin with the congregation (IK 2760). Balance joining the congregation with not skipping parts of the prayers such as Pesukei d'Zimra (IK 6583). Prayer and study should combine mind and heart, each inspiring the other (IK 2251). Steady attention to daily conduct matters more than grand preparations (IK 3310). A mashpia can guide one's inner service (IK 7222). On calm prayer versus the duty of teaching students, see IK 7897; on unwanted thoughts and focus, IK 6710.
See also: Prayer – Concentration and distracting thoughts; Prayer – Praying with a minyan and shul life.