When using @mouseover, it’s important to include other conditionals, as otherwise, whenever your mouse is over anything, it will evaluate as true. If what your mouse is over isn’t valid for the spell, the macro will just fail at that point. (Similarly, using the empty conditional for the target, will also prevent the macro from every checking for a focus if you have any target, regarless of if tricks of the trade works on them.)
So, you’re looking at something more like this:
#showtooltip
/cast [@mouseover,help,nodead][help,nodead][@focus, help,nodead] Tricks of the Trade
Edit: The above macro will do nothing if you don’t have a friendly mouseover, target, or focus. It will also show a “?” icon in those cases. So, you may want to add an empty conditional at the end as a default action. (If everything else fails, it will bring up a cursor for you to click a target.) If you don’t want that to happen, but always want the icon to show, then just add “tricks of the trade” to the #showtooltip line.
So, here are the two options. First, with the default action fallback:
#showtooltip
/cast [@mouseover,help,nodead][help,nodead][@focus, help,nodead][] Tricks of the Trade
And then without it and the modified #showtooltip:
#showtooltip Tricks of the Trade
/cast [@mouseover,help,nodead][help,nodead][@focus, help,nodead] Tricks of the Trade