Here are a few things I try to keep in mind when designing a new wandering monster chart.
Pick Dice Wisely
I don't want to hunt for special dice every time I need to dice up a wandering monster., so I pick what I'm sure to have at hand. For D&D that usually means a 1d6 or 2d6 chart. For a more percentage-based game like Encounter Critical I might go with one or two ten-siders. And I try not to pick too many dice, so that I don't have to stop and add up results. I often go with two or three dice, so I get a nice bell curve type distribution. That way I can put the most common encounters in the middle of the chart and the rare weird stuff on the ends.
It's Okay to Reference Other Charts (sometimes)
Often I will include an entry that references another wandering monster chart, just to expand the possible options. Like on a level 1 chart I might include "roll on the level 2 chart" or for a forest next to a mountain range each chart might give a small chance that you should instead roll on the other terrain chart. That way you occasionally get hill giants on the plains or swamp beasties in the hills. Occasionally I will put an entry on the chart like "Roll on the Fiend Folio instead", but I try not to overdo that as I don't want to flip through too many extra wandering monster charts.
Not All Entries Have to Be Monsters
The original Traveller wilderness encounter charts included events on them like avalanches and blizzards. In dungeons I often add sound effects or odd smells to the chart, or events like a gust of wind that might blow out torches. Non-combat encounters with ordinary rats and snakes and such also appear on many of my dungeon charts. Things that players can interpret as omens also make interesting entries.
Not All Entries Have to Make Sense
Sometimes I put a monster on a wandering monster chart specifically because it doesn't appear anywhere else in the adventure. I just want a small chance that a leprechaun shows up, you know? And one time I put the smell of freshly baked cookies on a dungeon chart, without there being any good reason for it.
Keyed Encounters and the Chart Should Interact
Say hex 1234 is deep inside the Forest of Doom and according to your key that hex contains a massive warg lair. It only makes sense to include wargs on your Forest of Doom wandering monster chart. For extra fun, I sometimes make a note on the wandering monster chart "remove from chart if warg lair in hex 1234 cleared". Or maybe I'll note that the result "ogre" really means "ogre from room 23", in which case if the PCs kill a wandering ogre then room 23 becomes ogre-free. Another thing I sometimes do is put multiple examples of the same monster on a chart. Say level 2 is mostly devoted to a bigass gnoll lair. I might make an entry for an alert and agressive gnoll patrol, another for punkass gnoll teenagers sneaking off looking for trouble, and a third for some grumpy gnollwives heading to or from the local watering hole with big clay jugs.
Anybody else got any good tips for wandering monsters?