The article above provides very clear explanations to the hollow-stiff conundrum. Strategies were coherent, common sense approaches to straightness. Also consider suppling exercises: poll flexions/counterflexions and neck bending/counterbending (depending on the direction you are traveling) to elasticize the stiff side, fostering better balance in the horse’s body condition, as well as his way of traveling.
Horses, of course, are mirrors of their riders. If we are stiff or if we sit crookedly, we will develop our horses’ bodies asymmetrically, resulting in stiffness, crookedness, and imbalance. Body work for riders is essential- when stretching, make sure you relax into the stretch and allow space to develop in your facia and your joints. Never force a stretch. Breathe and focus into the length and space you are creating. Side planks are fabulous for developing a strong, supple core to aid you in sitting in harmony and straightness with your horse. If your core is strong and balanced, you can influence a crooked horse to move more in balance, instead of abetting his asymmetry. Practice simple balancing exercises, such as standing on one leg with your eyes closed. Work up to 30 seconds. Use a balance board for 10-20 minutes/per day. The constant small shifts needed to keep the board level give your body feedback, which translate to the micro-adjustments a quiet, supple, balanced rider performs in the saddle.