I know cement based grout needs to be mixed, slaked and then mixed again, but I've never heard you should also do that with brick mortar. I can see no reason why that would do any harm, tho.

If you find your brick mortar dries up too quickly, it's because the dry bricks are wicking moisture out of the mortar. What helps a lot is to add hydrated lime to your mortar when mixing it. The lime hangs on to water and prevents it from being wicked up by the bricks, making your mortar remain workable for a much longer period of time. Adding lime will weaken your mortar because it reduces the percentage of portland cement in it, but the hydrated lime you add ends up turning into limestone (calcium carbonate) with the passage of time, and in most cases mortar as strong as limestone is plenty strong enough for residential applications.