One macro that every WHM should have:
/p <lastst> targeting <t>
The blank line is important for this macro and I'll explain why later. When you take the macro as-is (including the blank line) it will bring up a <stnpc> selector. When you pick an NPC, the macro will target the person at the top of that NPC's hate list (whoever is tanking). It's a great way to get a gauge on who is likely to be taking damage in the imminent future. The "/p" line will not be executed because of the blank line preceding it, which is generally good. People generally know who has hate, and don't need you telling them every few seconds. (And as a career WHM, I abuse the hell out of this macro. So people would get really annoyed if it always reported in /p chat.)
Now if you're in a fight where hate is particularly unstable, that's where the extra line comes in. If you add a "/wait 1" where I left a blank line, it will also report in party chat who has hate. This can be really handy, say for example if your tank goes down on an enemy that casts spells, and you need people to kite it if they get hate. It's possible someone might die and hate will change, and people won't realize they have hate right away, especially if the mob starts casting on them before it gets very close. If your macro announces it in party chat, they're more likely to run away and survive.
Thus if you set up the macro exactly like I have above, you can control whether it announces in party chat or not by adding/removing the "/wait 1" line.
Really that's the only macro that I'd say really makes WHM any easier. The rest of my macros are all for a single spell or to swap equipment. The only other advice I have is get good at targeting. The faster you can target someone, the faster you can start healing them.
Another advice is to keep your targeting methods in mind when choosing how your spell macros target. For example, I use mainly a mix of <stpc> and <t> in my white magic macros. I use <stpc> for almost everything; this is because I often have the person I want to cast on targeted already so all I do in that case is hit the macro, then hit enter right away. It's not much slower than <t>. In fact, the main reason I use <stpc> instead of just <t> for most macros is so that I can hit the macro before I actually want the spell to start, such as while my previous spell is still casting. Then I have the subtarget selector up and ready for me to hit enter as soon as I want to cast the next spell. However, there are some spells where I won't necessarily have enough foresight to get the <st> up before the spell is ready to cast, sometimes I just need to react quickly and even having to press enter one extra time costs too much. The main examples I can think of are Flash & ailment removal spells like Paralyna. For those spells, I use <t> to just always cast on whatever I have targeted. I've heard a lot of people swear by <stpt> and <stal> but they simply weren't around when I trained and I've gotten used to playing without them. When you get good at targeting people, <stpc> is actually faster than those two options.