decisions
In tactics, the most important thing is not whether you go left or right, but why you go left or right.
- unknown
Playing a game is all about making decisions and seeing how they turn out. The more options you have and the better you can trace a specific result down to a specific decision, the better. Wargames are no exception from this. It is about making plans, anticipating their outcomes and deciding. Thus, your scenario should not limit players' options but rather provide some interesting situations which have more than one approach to deal with.
Make Decisions Possible
Always leave room for decisions to be made by the players. If I have to move unit A to spot X and attack the enemy bunkers with the flamethrower set up just for that specific task my role as a player is reduced to moving dice and miniatures - I'm soon getting bored.
So do not come up with some kind of masterplan how to solve this scenario. Instead your setup should allow for different options, each with its own advantages, drawbacks and trade offs.
Decisions vs. Dicing
Whenever your scenario description contains elements of random determination, look for a way how to replace the diceroll by a decision. In the best case, this decision depends on both players and in the very best case the decisions is made on the tabletop. Players know better than dice.
What does this mean?
Well, assume that your scenario includes reinforcements for one side (let's say its the British). Now, the Brits get reinforcements - but when? You could say: "roll a d6 at the start of each turn. If it's a 5+, the British get their additional troops". But you could just as well say that the reinforcements appear whenever the British player wants but after they have appeared, the Germans get to release an artillery bombardement. Now, using reinforcements has become an element of planning and deciding instead of dicing.