Sorry for replying to this sooo late, I just saw you didn't get much feedback and thought I should add my little two sense. :)
Getting your horse to round up for proper collection does take work. A lot of work. Hopefully your horse already knows the basics of collection and can do so while oncontact (Like for Hunter, lets say!).
In Western Pleasure, our goal is to keep that collection, with a relaxed headset, loose rein, and moving with cadence at a gait natural to his body. Some horses will move faster at the jog, because they have a longer stride.(Remember that passing a competitor is not a sin, your horse is unique!) This collection actually makes Western Pleasure quite a hard sport.
In recent years, judges have been trying to steer the look to the poll level with the wither (or slightly above it) with the muzzle pointed slightly out. Unfortunately, there are still those who have penut rollers and drag their heads on the ground. This fashion is getting booted more so, but its still not completely popular for the complete natural look.
Therefore, to stay competitive, the poll should not go any lower than the horse pictured. And the muzzle should be where this horses' is, or slightly stretched out. Also keep in mind that your horses' headset should be the most natural for his build :)
Onto the collection. Your horse should be rounded, with it's bottom tucked. The picture you had was a great example. A way to be able to tell is if he feels heavy on the bridle, he's on his forehead and not using his hind or rounding his back correctly. Also, if he isn't rounding correctly, at the lope his back legs will extend past his hind end way too much.
It will take time and a lot of work to get them to round, stay rounded, and move correctly on a loose rein. I ask my horses to round by taking slight contact with their face, tucking my bum (turn your pelvis only and sit on your pockets) while I open my hips and wrap my legs around them, as if I were hugging them (think, lifting their back.)
A great exercise to stretch a horses' back and to make them feel comfortable, is to take your hand on their belly, slightly behind where the cinch goes, and run your fingers toward eachother and touch, then back out. If no response, apply a little more pressure until you can visually see them lift a little in their back. You can do this out in the field even, it's a great stretching excersize that will make them feel more comfortable while under saddle and asked to round (and they will begind to be able to hold it longer.)
When you start asking for collection, your horse may arc his neck too much, and may tip his muzzle behind the verticle. This is okay at first, as it takes time to get him to stretch down and relax when first teaching them these basics. Always remember, that as soon as they round and give you their head, release. Keep asking, focusing on light cues (if light "asking" doesn't work at first, go to a little more of "telling," then "demanding." Never go straight to "telling" or "demanding" or your horse will never get light in the bridle or cues!)
Eventually, as your horse understands the cues and gains the muscle to lift his back and tuck his hind end, you will be able to start stretching his headset out and he will hold it at a natural level and angle for his build. If he drops too low, do NOT pull on the reins. Reins mean give your head, i.e., down. Squeeze with your calf and his head should come back up. If his head comes too far up, lift with the rein and use leg/seat cues.
Always remember to tuck your bum when asking for him to tuck his hind end and round. Your body tells his body what to do (: To ask for collection and moving forward into a gait, like from a stop to a walk, make sure you open your hips as well (think, move your knees off of his sides, along with tucking your bum, and hugging him with your calves) as this will push momentum forward and he wont take off on his forehand.
Its a lot of work, but the end result is beautiful and natural. Judges notice it and they love it. My filly has mayyybe 180 days on her (including shows) in the course of 2 years and she wins literally everything we enter in. Our 12th ride we were placing above horses under saddle for 5 years. If you stay consistant with it you will be so unbeatable!
Also, I do not use much training equipment. I've only used long draw reins on her a couple of times for developing the lope, and shoulder control. Then I've only used side reins on my dressage horses to develop back muscles - I don't like using them on WP horses, or much at all.
Lunging exercises are great for development. I use a rope halter for so much more feel, and use my body to tell their body where to go. I have only used a 48" dressage whip on horses who need encouragement to tuck their bottoms and drive in the trot or lope, by bringing the whip in completely parralell to the ground at the back of their gaskins, but never touching them. It keeps them from getting on their forehand, or throwing their back leg out too far (in the western pleasure lope, ideally it shouldn't extend too far behind in order to stay rounded and slow. The back legs should drive forward, then come back to being around the position of a stopped lined up hind)
I train in a JR Cow dogbone bit with a copper roller, it has snaffle action first and then slight leverage. This helps me pick up whatever body part that I want in a horse and move it - your hands just must always go up, and never back (causes the bit to collapse around the jaw. It is a gag bit and must be used properly to preform correctly). The moment you put it on a horse you notice a difference in them getting their hind under them. For shows I use a solid port bit, because she bumps herself. I'm considering getting a Bob Avila swivel port for shows, because I love having independant rein action.
Another good exercise for control and collection, is sidepassing! At the stop, walk, trot, and eventual lope. A side pass or two track is phenominal, since your horse should always be slightly arced on the bend of the circle - and yes, the arena counts as a very large circle. Having your horse sidewind down the rail at the lope is unfavorable, because the horse is using the rail as a stop and not using his body correctly. Unfortunately, a lot of the competition does this so most horses don't get docked. My horse does not and we almost always place above a horse who is going down the rail facing it, unless my young filly accidently drops a shoulder (she's slowly developing her horrible balence, poor girl!). Teaching your horse to sidepass teaches them to yeild their shoulders, haunches, and lift their back. This is a trick we can use,(once your horse is light enough for no rein action, but correct body position) on the rail to get effortless collection and to slow their gait.
While training, pick up the inside rein a bit and use the inside leg in the corners of the arena to get them to arc and drive into the corner. This keeps them from dropping a shoulder and getting onto their forehand. Circling in every corner using this method also helps. Do not pick up one rein over another using a solid curb bit, because it shifts the position of the bit in the horses' mouth and gets them off balence.
To get your horse on his hind end, I do a lot of loping at first with good stops, a rollback, and lope off. He has to collect for the lope, stop when I tell him too by dropping and using his hind end, turn on his hind, then push off with his hind. I warm up with extended trotting (posting) to stretch their backs, then move into this. Once their stretched and then using their hinds, their ready for collected work.
Also, having mirrors in your arena, having someone video tape you in the center of the arena for you to review, or going to a trainer are very reccommended to make sure you are performing and training properly.
Hopefully this helps a little, good luck in the show pen! ;)
Mother to three amazing rescue babies<3 ~Pele ~ 4/5 year old palomino appy filly.
~Papillonn ~ 20 year old arab gelding. ~Ruby ~ 3/4 year old flaxen chestnut BSP filly.