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There are many Buddhist lineages in Tibet and all of these have variations on what they consider to be preliminary practices or ngondro. So in order to answer your question generally you have to look for preliminary practices that are common to all of the main lineages - Kagyu, Sakya, Nyingma and Gelugpa.
The 'common' or 'ordinary' or 'outer' practices are generally considered to be the Four Thoughts that Turn the Mind or the Four Reminders. These are more philosophical musings that prepare the mind and motivate the person to continue their practice:
A poem by ChΓΆgyam Trungpa Rinpoche on the Four Reminders:
Joyful to have such a human birth, difficult to find, free and well-favored.
But death is real, comes without warning. This body will be a corpse.
Unalterable are the laws of karma; Cause and effect cannot be escaped.
Samsara is an ocean of suffering, unendurable, unbearably intense.
It is in the uncommon or extraordinary or inner practices where there is great variability from tradition to tradition but the following are fairly common:
These preliminary practices are considered essential before journeying into the intermediate and completion stages of Vajrayana which, traditionally, cannot be done without a qualified teacher.
Recommended reading - A Beginner's Guide to Tibetan Buddhism