They are usually made up with suffixes or endings, added either to the full name or to another diminutive:Originally Posted by love.angel
/-ка/ Танька, Петька (but it's childish and a bit rude)
/-еньк, -оньк/ Оленька, Алешенька, Лизонька,
/-ечк, -очк/ Васечка, Вовочка
/-ха/ Леха (Алексей), Антоха (Антон), Ксюха (unceremoniously)
Often used these endings (but I don't see a system here):
/-ша, -жа/ Антоша, Ксюша, Олежа, Сережа, Маша, Паша (Павел)
/-ня/ Сеня (Семен), Таня (Татьяна)
When you see consonant+"я" it's always a diminutive form, IMHO.
Коля, Саня, Оля, Таня, Вася, Петя, Федя, etc.