Just click the button again that you used to summon your mount. For easy access keep this button on your hotbar. Also your mount has its own inventory window, on your character panel (mounts tab).
In this tab, you can select which mount you want to ride, and if applicable, which mount you want to use as an 'appearance' mount over the top of that (if you have multiple mounts, they will always show up in this window). The only except is account bound mounts, these can be removed from this window, and placed in your physical inventory, and then placed in your shared bank slots.
Edited, Oct 5th 2013 11:49am by Dyadem
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Things I sometimes play...
"What do you want to be when you grow up?"
"I want to be a unicorn!"
"Awww, why's that?"........
"So I can stab people with my face."