The Prophet
would commence his recitation with many kinds of du’as (supplications) in which he would praise Allah the Exalted and glorify and extol Him. And he also ordered the Muslims to do likewise. (Bukhari, Muslim, Ibn Abi Shaibah)

One can say any one of the following supplications after the first Takbeer (Tahreema):

1. Sub’hanaka-Allahumma wa bi’Hamdika wa tabaarak-asmuka wat'ala jadduka wa laailaha-ghairuk You are Glo3rified O Allah, and praised; your Name is Blessed; your majesty is exalted, and none has the right to be worshipped but You. (Muslim)
2. AllahuAkbar kabeera wal’Hamdulillahi katheera wa Sub’haanAllahi bukratan wa Aseela Allah is the Greatest, very Great, praise be to Allah, again and again. Glorified is Allah morning and evening. 
3. One of the companions commenced with the dhikr below, upon which the Messenger of Allah
   said: ‘Wonderful for it (the supplication) is that the doors of heavens were opened for it.’(Muslim and AbiDawood)
4. Alhamdulillahi Hamdan katheeran Tayyiban, Mubarakan feeh Praise be to Allah many pure, blessed praises. Another Sahabi commenced with this to which the Messenger
   said: ‘I saw twelve angels competing as to which of them would take it up ‘. (Muslim, Abu ‘Awaanah)
5. He would say Takbeer (AllahuAkbar), Tahmeed (Alhamdulillah), Tasbeeh (SubhanAllah), Tahleel (Laailaha-illAllah), and Istighfar (Astaghfirullah) ten times each and then say ten times 
6. Allahumma-ghfirlee wahdinee warzuqnee (wa’aafinee) O Allah! Forgive me and guide me and give me sustenance and (overlook  my sins). 
7. And ten times: Allahumma innee a’oodhubika min-addeeqi yawm al’Hisaab O Allah! I seek refuge with You from the distress of the Day of Account. (Ahmad, Ibn Abi Shaibah, AbiDawood and Tabaraani).

Source: AsSlaah, pg 68-69