This is a good article on exercises you can do from the ground to help a horse develop proper muscles to aid in self carriage. I particularly like the Lumbar Tuck... it's great for stomach, back, and hindquarter muscles. Click the link to read the entire article... below is an excerpt.
The Fan
Place four or five poles so they fan out and away from a central resting point on a half 44 gallon drum, stack or five tires or similar object (Figure 1). Begin by leading your horse in a larger circle which includes only the outermost edge of the pole fan. Gradually decrease the size of the circle towards the center of the fan. As the circle decreases the poles become closer and higher. If your horse becomes anxious or starts knocking poles, increase the size of the circle until he can easily cope with the pole distance and height. Be sure to work in both directions.
Cavaletti and the Climb Through/ Stretch Through
Walking through cavaletti is an excellent way to develop your horse’s core strength and flexibility. If your horse is not familiar with cavaletti begin slowly. Start by walking over one cavaletti set at the lowest height. When your horse is comfortable walking over this, add another. For most horses the comfortable distance between cavaletti is around 90cm: adjust as necessary for longer or shorter striding horses. Add cavaletti until your horse is comfortably walking over seven or so. Next, raise the fourth cavaletti to medium height. Then raise the sixth cavaletti to medium height, then the third, etc. Raise the cavaletti one at a time and only as your horse develops confidence. It may be necessary to shorten the distances between these raised cavaletti by 5-10cm. For advanced horses you can gradually set all the cavaletti at maximum height or, better still, create a Climb Through/Stretch Through.
You need two rows of cavaletti to make the Climb Through/Stretch Through (Figure 2). The first row consists of four cavaletti set close together (80-90cm) at maximum height. The second row consists of three cavaletti on the lowest height and set far enough apart that your horse has to lift and reach with his front legs (95-110cm).
The Shoulder-In
In this lateral movement your horse’s forehand travels on a path inside that of the hindquarters, with even bending along the spine. The degree of bending is the same as you would expect on a small circle. To introduce the shoulder-in begin by having your horse walk a small circle (6-10m in diameter) around you (Figure 3a). Keep a soft or loose hold on the lead-rope and point a dressage whip or carrot stick toward his quarters. After three or four repetitions allow him to take a few straight steps along the side of your work area before bringing him back on the small circle around you. Repeat this process until your horse anticipates moving onto the circle after a few straight steps. Once your horse is familiar with the procedure you can ask for a step or two of shoulder-in.
The next time you allow the horse to move along the straight side simply step toward his shoulder and bring the whip toward his flanks (Figure 3b). Initially it may be necessary to ‘gently feel’ the lead-rope to keep him looking toward the center of the work area. Ask only one or two steps before resuming the circle. Gradually build up the number of shoulder-in steps between circles as your horse becomes more familiar with the movement. The shoulder-in is physically and psychically demanding and must be developed harmoniously and with trust. Merely pushing or pulling your horse into the required form will have no gymnastic effect. Horses and riders with necessary equitation skills should practice the shoulder-in under saddle.
Stretches
Perform the following stretches during breaks between the above exercises. This allows your horse time to catch his breath after the metabolic demands of strength training. To be effective each stretch needs to be held for three minutes. This is the time it takes to effect ‘muscle memory’, where the muscle cells come to recognize the stretched position as normal. Initially this may be difficult for your horse. Take time and develop the stretches over several weeks. Remember a gentle stretch held for a long period of time is more effective than a stronger stretch held for a short period of time.
Wither lift. Stand level with the girth facing the side of your horse. Bend your legs and apply the fingers of both hands to the midline of his undercarriage, directly behind area of the forelegs, in an upward direction. The withers should rise up noticeably. ‘Stoic’ horses, those who are desensitized to touch due to pressure from the saddle and girth, may require a very firm upward pressure. Use your fingernails if necessary but remember to use the least pressure possible – don’t you do the work for him.
Lumbar tuck. Only perform this stretch if you feel confident your horse will not kick. Stand to one side of your horse and at the hindquarter facing toward his head. Place one hand on each side of his hindquarter, level with and around 10cm to the side of the point of croup. With firm and increasing pressure slide your index or middle finger back to the point of buttock. The lumbar area will rise and the hindquarters will tuck.
