Don’t force it
There are times when a change has to be made, but making the change will be tricky because it’s being retrofitted to existing geometry. The temptation is to force the change in with minimal alteration - this often results in brushwork doesn’t quite fit, or becomes very untidy.
It’s key to remember that it’s generally easier to delete’n’add than modify. In cases where you need to significantly alter geometry that you hadn’t planned to (add a staircase between two floors in a confined area, for example), and making minimal changes is going to get messy, then delete slightly more than you need to and work from there.
Understand basic architecture
Know how buildings are typically constructed and how they stay up - use this to make your own creations look more convincing. Spikes are out, as are rooms with ‘V’ shape corners. Large and open platforms need to be supported somehow. Old buildings generally use wood rather than metal and concrete. New buildings rarely use stone and mortar. Pillars aren’t placed randomly - they traverse between floors. If a building is jutting out of some sort of pier or rockface, it will need some sort of support and way to keep it up. Strip lighting is going to be rare in a 1940 Spanish villa.
Understand common sense
A walkway far from the ground in a warehouse is likely to have a handrail. Very tall ladders are likely to be caged. Longer climbs will often have platforms to break them up. A large interior area will likely have some large entrance/exit, and not a little person-sized door. Sea-level is considered constant. Building walls at street-level are typically dirtier and dingier than the floors above. Litter is a city thing, not a country thing. Grass does not grow underwater. Grass does not grow on metal, brick, or even concrete.
Understand weather
If the sky is bright, then so is the sunlight. If the sky is dark, then so is the moonlight. However, moonlight is not pitch-black.
Fogging is based on humidity and atmosphere. Try to match the colour to the skybox - in the worst case, it’ll highlight the seam where your map geometry ends and the skybox begins.
For a very warm, stick, humid feel, bring the fog right in and make it faint and light. For a clearer day, push it all the way back (leaving a subtle haze over distant structures.) At night, a very faint grey works well.
Note down problems
If you’re testing your map, and you spot an error or problem - write it down! You might think you can remember it, but trust me, you won’t remember everything. If writing is too much of a chore, take screenshots instead. Fix any problems as soon as you can.
There is no checkmate
There should never be a checkmate or stalemate condition, by which no matter what the player does, they will always lose against another player who is in some advantageous position. There must always be a way to counteract whatever another player is doing. Multi-player gaming is about reacting and trying to out-think or out-play the opposition, so you must let them to this. Single-player gaming is about finding solutions to problems and beating the game, rather than players.
No matter how bad or difficult a situation looks, there should always be something that can be done that could end in success. If there is no solution or way out to a situation the player has got into, then the chances should be that the player is already dead.