My advice is to always remember that they are children finding boundaries, learning and living life.  They will make mistakes as all of us do.  You are investing in their future and they will grow to understand that, may not today or when they are 18 but later in life.

The hardest part for me was knowing that I invested so much love, time and energy in them and they could be stripped away from me in a moment.  I had to remind myself that all though this is a child parent relationship it is just like any other relationship.  We invest our time with people at work, friend and neighbors and all of them can be gone tomorrow but what they have learned from you (everyone learns something from everyone they meet) will not just go away when they are not around you.  It is just like a child they will always have a piece of you because you helped shape and mold them into who they are.  They may not see it but in the future they will.

Hope this might help a little!  You are very special person for stepping into this role!