There are 9 of you and usually only 1 of them.
No rundown should ever fail to get an out. That they do is usually the result of one of three preventable errors.

• Your whole team doesn't get involved.
• You execute too many throws.
• You leave yourself open to errors - through poor positioning and poor ball handling.

Charging and throwing in a rundown situation is different than when you are doing a fielding throw...

• Keep the ball ahead of the runner - always chase him back, the most likely tag area is 3-4 strides from the previous bag.
• Keep the ball in view - when you try to hide it from the runner, you hide it from your fielder.
• 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 - catch all relays on the tosser's throwing arm side, never throw across the runner's path.
• Don't block the basepath unless you have the ball - as soon as you've made a relay, rotate back and let the next fielder get a good view.

Check the coverage diagrams below to see who should be where, and why.
Black lines are direction of player movement.

Stealing Home

Primary is the Catcher - get the ball to him first. Third moves up the line a couple of strides to take away slide opportunity. Pitcher is closest fielder, First backs him up, then Second. Shortstop is closest to third base, Left fielder backs him up on overthrow. With trailing runners, Center and Right fielders move to bags, otherwise all the way home.
Stealing Third
Primary is Third baseman - get the ball to him first. Shortstop and Pitcher break to third with runner (but off the basepath). Left field is the back-up. Second is backed up by Center and Right fielders. Catcher moves up the line a couple of strides, with First backing him up.

 

Stealing Second

Primary is Second or Short (depending on who would normally take the throw), Center backing them up. First moves up the path to take away slide, with Pitcher, Catcher and Right backing him up. Third moves off his bag toward second and Left fielder comes in to back-up third.