Of course, a pope may change these, as in the case of St. Hildegard of Bingen and others. Also, martyrs are sometimes canonized without miracles and other saints if there has been a long history of veneration.
The following are the basic steps to becoming a saint: http://www.freerepublic.com/focus/religion/1916620/posts
1) The candidate must be deceased for a minimum of five years
2) Even in our modern, fast-paced society, the candidate must have lived a life of extraordinary asceticism, self-denial, faith, good works and inspirational virtue, often helping the poor and sick
3) The candidate must be recognized for two posthumous miracles...verified by a scientific board appointed by the Vatican
3) Once it is determined that the candidate has led a life of holiness, he or she is declared a Servant of God and the investigation of their candidacy begins
4) A postulator is appointed on the candidate's behalf, undertaking an exhaustive investigation into his or her life, works, writings, sayings, family life, early history, etc. – digging for any small thing that might be inconsistent with tireless faith and Church teachings
5) Witnesses (if still alive) are called to appear before a church tribunal and further evidence is gathered
6) A positioning paper presenting the case of the saint is handed to the Bishop
7) If the Bishop agrees with the evidence, he passes on the report to the Vatican's Congregation of Saints
8) If the Congregation and the Pope agree that the person lived a virtuous life, the candidate is named a "Venerable."
9) This completes the investigation of the candidate's earthly life. Now, the Congregation undertakes the investigation of the two posthumous miracles, if they have occurred. If not, they wait. The first miracle earns the candidate beatification, the second assures sainthood.
10) Miracles are intensively scrutinized by both religious and scientific authorities. Medical miracles are examined by a board of five doctors who must unequivocally determine that no other possible explanation for a cure exists.
11) All cures must be instantaneous and complete (One potential candidate's miracle – restoring the sight of a blind man – was rejected because the sight was only 90% restored). In the case of cancer, a ten year waiting period must assure that the patient doesn't come out of remission
12) If the high standards for miracles are met, the Pope bestows the title of saint on the candidate