You'd have to decide which territories are most likely to be attacked or most important to defend, and use some troops in the backfield.
The most important thing to defend is my biggest bonus..., but not just any territory, all of them (and the biggest bonus usually has the most territories.
Richard's idea (not allowed further reinforcements) has potential, but would require a pretty big change to the way WL works (selectively disallowing deployments is currently impossible I think).
Besides, it also creates a bunch of corner-cases. Let's say you're doing a Market Garden; Airdrop (or whatever it's called) some troops at the corner of a few big bonuses, and multi-attack with a huge stack towards the same point. If both attacks succeed, would reinforcements still be disallowed? And on which territories exactly?
I wish there was a way to playtest new ideas and see how they work out without Fizzer having to implement them...?
I've been thinking, would allowing a "Dungeon Master" for test games work? Someone who is not a player himself, can see everything and can change the state off the board at any time, in any way?
Then you could have a couple testing games where the DM (on a piece of paper, or in private chat) keeps track of the number of Airdrop cards, is told which card a player wants to play (and when), then manually performs the effect by manipulating the board (and telling the affected player what happened).
This would take quite a bit of code to implement, but once in place, new suggestions could be play-tested properly before being implemented for real.