We use Tact-Tiles. This allows us to just add to the map no matter what direction we're going in.
Depending on the likelihood of us having another encounter in the area already drawn, we either erase those tiles, or keep them off to the side to avoid having to re-draw that area.
Other than a bit of staining of the tiles from the dry erase markers, I've been very happy with them as we never have to worry about running off the edge of the battlemat any more.