The Trust of the Imams in the Outcome of an Istikhara
al‑Imam al‑Sadiq (‘as) said: "It does not matter to me that when I seek the best from Allah, which way it falls (whether it results in ease for me, or causes me difficulties)." 
The true believer has no doubt that Allah, the Glorious, only wants the very best (for His servants). He knows that Allah (swt) is the endless ocean of Mercy and that it is far from His glory that His servant calls upon Him and asks for the best, and He makes him wait.
From another angle, the true believer has a firm belief that Allah has the power to open up one's path to prosperity and ease.
The true believer knows and understands that what is good for him does not always lie in ease and comfort; and sometimes, it is possible that what is truly good for him may be in observing patience and enduring trials and tribulations.
In light of the three points mentioned above, after the true believer supplicates and sincerely asks for the best from Allah , he must not despair or become disheartened. Whatever actions he performs afterwards must be discharged with complete satisfaction and conviction of the heart.
One Must Believe that an Istikhara will Solve the Dilemma
al‑Imam al‑Sadiq (‘as), said: "Whosoever asks Allah for the best and is pleased with what Allah has decided for him, then without doubt, Allah will secure the best for him. " 
According to the above‑mentioned hadith, the pleasure of the person will be in that which Allah decides after the person has asked Allah for what is the best. This is one of the etiquette and conditions before one performs the customary Istikhara.
Before one asks Allah for the best, it is imperative that attention is paid to the following point:
"There is no believer that calls upon Allah except that He answers the call. " 
That which a person supplicates to Allah for is either granted in this world ‑ sooner or later, or it will be given to him in the next world; or something bad that was going to happen to him which was impending to occur, will be averted 
Opening of the Doors of Goodness
al‑Imam al‑Sadiq (as) said: "Whosoever asks Allah, the Glorious and the High for the best a single time and is content with what Allah has decided for him, then Allah will definitely make that which is good for him come about. " 
One of the most important etiquette of seeking the best from Allah is that one must have certainty that He will decide the best for His servants and that He has the power to give or withhold whatever is best for His servant.
In short, the servant must have complete satisfaction and peace of heart that after performing the Istikhara, whatever is truly the best for him is the only thing that Allah will decide.
 Bihar al‑Anwar, Volume 91, Page 223, Hadith 330
 al‑Kafi, Volume 8, Page 241, Hadith 33 / Mahasin, Page 598, Hadith 1 / Wasa'il ash‑Shi'a, Volume 8, Page 63, Hadith 10094
 `Uddatul Da'ee, Page 34, Hadith 8 / Wasa'il ash‑Shi'a, Volume 4, Page 1084, Hadith 9
 `Uddatul Da’ee, Page 24, Hadith 8 /Wasa'il ash‑Shi'a, Volume 4, Page 1086, Hadith 8
 Fath al‑Abwab, Page 257 1 Mahasin, Page 598, Hadith 1 / Bihar al‑Anwar, Page 91, Hadith 256