Since many of us are to the point where we need to pay attention and cut down on the silly mistakes, which I'm guilty of I'll openly admit. I figured it was time to get started on a thread of helpful macros, and I'm going to start off with a great one that has saved my butt many times.
Type this into your macro creation screen:
/stopmacro [target=focus=targettarget,noexists,group]
/assist [target=focus,exists]
/attack
Now assuming I didn't screw something up in there

. That macro needs just one more thing to do to set it up. When you join a group and know who the MA(main assist) is going to be, target them and type /focus.
What this macro is suppose to do is make it so no matter who you currently have targeted when you hit this macro key you will automatically target the target of the person you put /focus on. And then you will start attacking, or you can remove the last line and you will simple target your focus's target, and not attack.
More to come, please add your own if you have any we should know!