With stains, calm and precision matter most. Remove any excess first, then blot the area gently, and only then, if needed, use a little water or a cleaning product. Rubbing and soaking almost always spread a stain.
When something spills, do not rub it. Press a clean handkerchief, napkin or paper towel onto it and let it soak up the liquid. Let mud dry, then brush it off. A water mark on dark wool usually disappears by itself once it dries and is brushed. Coffee, wine or sauce need more care, but they do not automatically mean a trip to the cleaner.
For light marks, dampen a clean white cloth with water: damp, not wet. Dab the stain from the edge towards the centre, without scrubbing, and keep switching to a clean part of the cloth to lift the loosened dirt. If you need a cleaning product, choose one made for wool and test it first somewhere hidden, such as under the lapel or on the inside of a trouser hem. An area that gets too wet leaves a tide mark when it dries, a visible ring where the wet area ended.
Greasy marks need a different method, because water alone will not shift grease. Blot gently, sprinkle on a little talc or potato starch and leave it for a quarter of an hour or so to absorb the grease, then brush the powder out. If a mark remains, use a stain remover made for wool, again after testing it somewhere hidden. Avoid wet wipes: they often contain fragrances and conditioners that stay in the cloth and can turn a small spot into a pale, spreading patch. When you are out, simply blot the mark and deal with it calmly at home.
If a stain is large, strongly coloured and has soaked in deeply, home methods will only spread it. Do not experiment; take the suit to a dry cleaner as soon as you can. Tell them what caused the stain and whether you have already tried to remove it, as that helps them choose the right method.