Tower Shield says "As a standard action, however, you can use a tower shield to grant you total cover until the beginning of your next turn. When using a tower shield in this way, you must choose one edge of your space. That edge is treated as a solid wall for attacks targeting you only."
Once you've chosen a side of the square you're in, that's it. Moving to a new square makes that side of the square you left no longer valid, but a side of the new square you've entered does not automatically get treated as a solid wall until you spend a standard action to make this new square edge be treated as a solid wall.
Stated another way, when you designate an edge of your square, it's not relative to you, it's fixed to the map.
^So if you Move (or 5 foot step) and then Standard Action(Take Cover), you should be covered.
Recently in my game a tower shield specialist wanted to gain full cover from a stationary ranged enemy using a tower shield. For some reason I thought that a tower shield had to be set to be used in this manner...
As previous posters have noted, it does indeed take a standard action to "set" the tower shield to provide total cover from one direction.
Which is another reason why enemy archers should be working around the flanks or rear of their target. <g>
^So if you Move (or 5 foot step) and then Standard Action(Take Cover), you should be covered.
If you do this, the archers could ready an action to shoot you while you're moving.
Indeed, the response to someone with a tower shield is to ready an action to shoot them when they start moving (and thus cannot maintain their cover).
Also, moving to the side can help quite a bit.
^So if you Move (or 5 foot step) and then Standard Action(Take Cover), you should be covered.If you do this, the archers could ready an action to shoot you while you're moving.
That's true -- good reason to have something like Protection from Arrows up (and it lasts a LONG time). (Also, if you move through the wrong squares and fail your Acrobatics roll or forget to do it you could get AoO'd before you can get your cover back up.) Of course, if you have Protection from Arrows up and are still doing the Tower Shield thing, advancing with a Tower Shield would probably be some kind of formation thing you are doing with teammates to advance a Shield Wall, and/or you are some kind of Stalwart Defender (although the latter requires some way of removing the Fatigue from your previous Defensive Stance, if you went that far with it).
Moving up the battlefield against archers behind battlements was exactly what the Tower Shield was designed for. The counter tactics mentioned effectively invalidates (to an extent) exactly what the tower shield is supposed to do.
Although if the archers are repositioning and/or readying then they aren't rapid shot and/or full attacking, so moving and setting a tower shield is not a completely invalid tactic depending on the archers.