Let's see depending on the wacker
Glasses - save the eyes
Long sleeve shirt and pants - save the arms and legs
Walk in the direction of - hmm forget...I relearn each time or just do it right after time....walk in correct direction to reduce the stuff hitting you even with the guard on.
Watch for cables and such like Cable TV boxes or old phone lines if still have those.
How thick are the weeds -
Watch deciduous or heavy stuff
I also have started using weed cutter grass whip. Has a serrated on both sides of an angled blade at end of a short-ish wood handle. Mechanical - swing it back and forth to cut through the weeds. But not A sling blade or kaiser blade