There's already an excellent guide for making macros and a catalog of the options at WoWWiki's Making A Macro page.
If you're an absolute beginner and want to get in on the helpful world of macros, go ahead and click that link, read that page (or at least make sure you understand how to do most basic functions) then come back here.
I'll be discussing habits that I personally use and have seen friends use to make life easier using macros. If you've ever said to yourself, "Wow, macros seem amazing! But where do I start?" then you've reached the right blog post.
Adding #showtooltip as the first line in a macro will let you read the spell tooltip by hovering over it. In a macro like the following:
/cast [mod:shift] Frostbolt; Fireball
...you will get to read the Fireball tooltip naturally, and the Frostbolt tooltip when you hold down shift. You won't have to open up the spellbook and find them to see how much damage they do or what their mana cost is.
Also, if you choose the giant question mark icon, it will show the spell icon naturally... and the icon will change! In the above example, it would show the fireball icon normally, and then the frostbolt one if you hold down shift!
Choose icons that stand out
If you have an ability that you MUST be watching the cooldown on, macro it, give it the #showtooltip line, and choose an icon that's bright and you can't miss. For an arcane or frost mage, a bright red or green icon will stand out wonderfully.
Keep in mind that if all your macros use bright icons, your bars will be overwhelming to look at, and nothing will stand out at all! I would only suggest three bright/noticeable icons at most. Two or even just one is further recommended.
Group spells that don't trigger the global cooldown
Trinkets, Potions, Arcane Power, Heroic Strike... These can be grouped together and used in one macro. My example is as follows:
/cast Arcane Power
/use Potion of Wild Magic
/use Mark of the War Prisoner
/use Mana Sapphire
/cast Icy Veins
/cast Arcane Blast
Only the final spell in this macro triggers the global cooldown.
Using this will activate my AP and IV, use my trinket and an offensive potion, restore some mana using a mana gem, then start casting an Arcane Blast. I named this macro "Combust" and gave it a bright red icon. You might guess why! Be sure your tank has adequate threat lead before using...
Use left and right mouse buttons for out-of-combat buffs
The abilities you don't have keybound (the very select few!) can be grouped by using [mouse:1] for left click and [mouse:2] for right click. Using those pesky paladin buffs as our example:
/cast [mouse:1] Greater Blessing of Might; [mouse:2] Blessing of Might
to take it one step further, we could remove [mouse:2]. That way, if you keybind it, or click it with a third mouse button, it will still cast something.
Melee and hunters should change all of their primary offensive abilities to have /startattack as the last line - or first line if the ability has a cast time. (I'm looking at you, Steady Shot!)
/cast Icy Touch
It's that simple!
That's it for now. Put em to use, find some more, and comment if you have any good ones I haven't posted!