From the BSN:
http://social.bioware.com/forum/1/topic/347/index/9629045
To prepare you for ME3 and two of the major conflicts, I can tell you what paths to take in order to achieve peace between the geth and to get both Krogan and Salarian support.
Geth vs. QuariansIn order to achieve peace between the two races, you need to get 5 to 7 points. These points are based on your ME2 and ME3 decisions. If you have 4 points or below, you cannot achieve peace.
-Rewrote the Heretics (0 points)
-Destroyed the Heretics (+2 points)
-Tali is NOT exiled (+2 points)
-Tali has been exiled/You did not do the Loyalty Mission (0 points)
-Resolved Legion/Tali conflict either using the Paragon or Renegade options (+1 point)
-N7 Mission: Save the Admiral on Rannoch in ME3 (+1 point)
-N7 Mission: Destroy Geth Squadron on Rannoch in ME3 (+1 point)
-Completed Legion's Mission in ME3 (or no peace)
You will need a HIGH reputation for the peace talks. If you rewrote the heretics in ME2 BUT achieve peace, you will gain more war assets, but there'll be less quarians. Also, if you rewrote them, you will face more geth than people who destroyed them, in which they'll face less geth.
Krogans vs. SalariansUnforunately, you can't save both Mordin and Wrex. One must die.
To Save Mordin: Convince Mordin to fake the cure. Wrex will know that you lied and will try to kill you, meaning you must kill him. Eve must die. You gain Salarian support, but lose Krogan support. In order to REGAIN Krogan support, talk to the new leader and he will not know the difference and give you support.
To Save Wrex: Mordin will want your help to cure the Genophage (yes, he wants to undo it). Mordin will sacrifice himself to cure the genophage. You will gain Krogan support, but lose Salarian support. In order to REGAIN Salarian support, save the Salarian councilor during the Udina Coup.
EDIT: I apologize. For the Krogan vs. Salarian conflict, IF you want to cure the genophage, you will need Maelon's data from ME2.
Sources: Leaks.
Hope this helps. If anyone wants to add anything or correct me, feel free!