You can poke the cake with a fork or toothpick before pouring the syrup. If you want a moist cake..try making this tres leches cake..it's yummy and super moist.
Take two trays asper the size of your cake pour the juice into each tray. and dip the cake untill all the juice is absorbed.then do the icing.(only add just enough juice to absorb or it will be difficult to manage)