Being flexible is key to being a better rider IMO. It's not only going to help you with mounting but in all aspects of your riding if you can be fitter, slimmer and more flexible. I stopped riding for years because of my excess weight, it was too hard to manage being on a horse at the size I got to before I decided "enough is enough" and I began a fitness and weight loss program. I modified my diet and joined a gym, consulted my doctor and a personal trainer, and did lots of exercise to improve fitness and flexibility, increase strength and attack excess fat. Lunges are great, as is using a variety of machines at the gym. One of the best I found for riding is the "glute press" which basically tightens up the buttocks. I did shoulder presses for upper body strength, ab exercises for core strength, and lots of cardio to shed the fat! Rowing machines are terrific for both cardio and strength.
Without a gym, the best exercise IMO is stair climbing.