I think a good way to help boost troop donations is to provide them with a half-back resource of the donated troop to the donator, rounded up 1. Pretty self-explanatory, but here is an example.
- If a Pekka costs 30,000 Elixir, the donator gets 15,000 Elixir back.
- If a Barbarian costs 25 Elixir, the donator gets 13 Elixir back.
- If a Hog costs 40 Dark Elixir, the donator gets 20 Elixir back.
You get the idea.

