RUNDOWN PRACTICE
Keep the ball ahead of the runner - always run straight at the runner regardless of where you start on the field, the most likely tag area is 3-4 strides from the previous bag.
Keep the ball in view & hold your arm steady - hand up by your head, lots of white showing, no fakes. (Minimum ball movement will actually make the runner less certain of your intentions.).
Make a short-arm, snap throw. - a wrist shot straight to the glove.
Throw beside, not across - The receiver should present a good target on the throwing arm side for the player with the ball, never throw across the runner's path.
Don't block the base path unless you have the ball - as soon as you've made a relay, sprint forward to the base you just threw at to let your back-up fielder get a good view for the next throw.
This is a good practice drill - You can keep it going for several relays but in game conditions you want to keep a rundown to 1 or 2 throws max.