There is a way to ready actions before the other side gets to act as you will. As someone else stated, its about defining when combat starts.
In dnd terms, surprise round is about "awareness." Let's take the crossbow and the closed door scenario that everyone loves.
The party heads to a closed door. There is a guy with a crossbow on other side of the door, who intends to shoot the first guy who comes through the door.
Now there are 4 ways this scenario can be played out.
1) Complete surprise. The pcs approach, everyone gets listen checks. Neither the party nor the crossbowmen detect the others approach. The PCs open the door, and everyone is surprised. You roll initiative, and play out the scene.
2) Crossbowman detects the party. With a good listen check, he knows someone is coming to the door, but the party knows nothing. At this point, the dm can rule combat has started. The crossbowman can do whatever he wants at this point, but keep in mind many of his actions might give the party additional listen checks. So he decides to stay quiet, and simply READY an action to shoot the first person to the come through the door. The fighter comes through, and gets a crossbow bolt in the chest. Now the party rolls initiative, the crossbowman goes right before the fighter as by readied rules.
3) Party detects crossbowman. He can't ready any actions, he's completely unaware. The party decides to stay sneaky and rush the room as combat has started for them. The fighter will open the door, and everyone else readies an action to move into the room as the door is open. Once their actions are done, the crossbowman rolls initiative, the other party members go right before the fighter, as by the readied rules.
4) Everyone is aware. Combat starts before the door is even opened, everyone get their actions, readied or otherwise, and you play out the scene.