What's the best treatment for pain in the sacroiliac and sacrum area?

Heat: Heat and antiinflammatories.

Sacrum pain: The best thing to do is plenty of walking with good posture, avoid lifting, and do flexibility exercises. Sometimes massage and heat can help too. Take an occasional anti-inflammatory and that can help too. In general, using your core muscles while doing pretty much anything can avoid stresses to go to the lumbar spine and sacral junction and that avoids further injury.... Good luck!