There are many solutions, I'll give a few suggestions:
1. Use fingers 1,2, and 3
2. Bring the RH thumb over the neck a bit (this tends to let the fingers arch more)
3. The first knuckle (on any finger) will get stronger over time if you are aware of it, and make an effort to Correct it's natural tendency to flatten.
4. Don't worry about the fifth string, I usually play the G with my 2,3,and 4th fingers like so-
As for technique the forearm, wrist and back of hand should form a straight line (or straight as possible) a little arch in the wrist is nessesary at times.
Hope this helps. Steve