Wow, if this is your first time to braid then you did a great job! Especially with his mane so long! Like sandsarita said, his mane needs to be quite a bit shorter. -no longer then a hands width, or about 4 or 5 inches. You want your braids to be small, even, tight, and as many as you can get along his neck (like, at least 30!). It's easiest if you use yarn and "sew" them together. There are different kinds you can do- button braids: the small "balls" or knot looking braids like what you have, or what's most common in the hunter ring, almost rectangular loops that lie against your horse's neck.
Another fun idea (esp with longer mane) is kind of a french braid that they do with draft horses or some dressage horses? Or I 've seen some western people do this thing that's kind of like banding, but it makes their mane look like a net? Google horse mane braiding and I bet a ton of fun pictures/ideas will come up!