My only piece of advice right now would be to avoid long, large, stretching shadows on tile-able battle-maps, and to preferably make them lighting independent.
That's the main problem with tile-able overland battle-maps for me, they're not as impressive as Dungeon tiles or regular battle-maps due to the lack of proper lighting - of course this could be avoided by having all tiles only go a certain way, but that's not very good.