The most recent incarnation of my mount macro includes combining it with another ability that is only used in combat. This serves three purposes: it cleans up my action bars, I can't accidentally hit my combat only ability out of combat, and I get a speedy hotkey to mount.
The ability I've chosen for all of my toons is a crowd control and/or snare removal ability. For druids this would be "power shifting" or shifting out of and back into your current form, expending a global cooldown in the process. Obviously shifting forms isn't something that burns a cooldown out of combat, but I'm just being consistent between toons. I've also got my mount combined with Cloak of Shadows (Rogue), Tremor Totem (Shaman), Lichborne (Death Knight), Fear Ward (Priest), Master's Call (Hunter), Escape Artist (Gnome), and Hand of Freedom (Paladin).
Right now everyone has 3-4 mounts they use on a regular basis: an Abyssal Seahorse, a flying mount, a ground mount, and perhaps an additional RP type mount. I know my Worgen have use an additional ground mount just for fun when they don't want to use Running Wild. So keeping all of these things in mind my mount macros look something like the following:
/use [combat] Cloak of Shadows; [btn:2] Black Stallion; [swimming,mod] Abyssal Seahorse; [flyable,nomod] Magnificent Flying Carpet; Running Wild
/use [nocombat,swimming,mod] Abyssal Seahorse; [nocombat,flyable,nomod] Swift Flight Form; [nocombat] Swift Zulian Panther; [form:1] !Bear Form; [form:2] !Aquatic Form; [form:3] !Cat Form; [form:5] !Moonkin Form; !Travel Form
As you can see out of combat the Abyssal Seahorse will only be used if you are swimming and using a modifier key, otherwise if you can fly it will use your flying mount and otherwise your ground mount. In the case of my Worgen I use a right click to use an alternative ground mount. Using a modifier will also use your ground mount over your flying mount, sometimes it comes in handy. In combat these macros behave just like the ability or abilities you combined it with.