Whatever it takes :)
Something fun (stickers) or healthy if that's enough 'currency' - if not chocolate buttons/sweets.
If they need it, they need it and if you can't get it down them just by telling them they need it or healthy incentives then an 'unhealthy' one it will have to be. It's fine.
It works really well and we didn't have to bribe, just said he needed to drink it, but I would have if we'd needed to. No problem certainly do for lesser reasons!!