Well your Grandmother was right. Yes you do need to stand up straight! Improve your posture, core strength and movement patterns to decrease pain and increase Function and Performance - here are a few tips to get you started:

The ABC's of Pilates - Alignment, Breathing and Core Control

Alignment

In standing  posture bring your head and ears back over your shoulders, balance the rib cage on top of (not in front) the pelvis, balance the hips level and straight (not tipped forward, tucked under, twisted or hiked up on one side) and stand with parallel legs, neutral knees (not hyper-extended), and your weight equal over the 4 corners of the feet. There is a lot that can go askew here but correcting faulty posture is KEY to decreasing compression and shearing of the joints, muscle tension, fascial dysfunction and pain.

Breathing

Practice a lateral and posterior breathe pattern; breathing into the lowest lobes of the lungs and wide to the sides of the rib cage. Avoid pushing or bulging the belly outward. Use a controlled exhalation to assist in gently drawing in, or "gathering in" the deep “navel center”. Lift the pelvic floor upwards and gather the deepest abdominals in like a corset, slightly tightening around the lower belly.

Core Control

As described above; the pelvic floor, inner most abdominal layer called the transversus abdominus, plus the deep spinal stabilizers called the multifdus, make up the core or the "inner unit" of postural muscles. Learning to engage these inner muscles to stabilize and support your pelvis, hips and low back leads to decreased back pain, better hip function and improved posture.

Mind Body Control

Have a system of checkpoints and think often about your posture. Become aware of your habits so you can work to re-align and re-balance your joints. The brain controls all of these systems so when you improve your awareness you can more easily improve your balance, strength, and movement patterns. Start with the brain, breathe deeply, engage the inner unit/core and move mindfully.