The tennis balls in a sock' therapy is great! Putting them in the sock just keeps them from rolling all over the house. It's super for hip flexor, back, and glute issues. Just play around, roll around on them, get them into those muscle knots!
A big part of proper posture is not just always focusing on it 24/7, but instead taking 15-20 min each day with eyes closed and a straightened spine. simply be aware of the alignment in your body. it will become easier to sit and stand straight over time!