How To Train Accuracy
The game of dart is all about hand and eye coordination. Some board manufactures also produce boards that have half the size doubles and triples.
Here is a picture of a dart bristle/sisal training board:
Picture is taken from: http://www.dartsupply.no (norwegian darts store)
Playing on this type of board will help you improve your game on standard dartboards.
If you don't have this type of board you can improve your game the following way.
Close grouping of darts is always considered a good thing as this shows consistency of throwing. When you throwing at doubles or triples sometimes darts block / cover a clear view of the target, focusing on a small area is then necessary.
If you have a sticker of some sort you can place this on the board on the target that you want to hit. Here you focus on the sticker an aim for that target, your focusing and eye coordination will improve.
Another thing to do if the darts block / cover your target is to move from left or right, this will change the view and you have a clear sight again. Professional players do this, if the darts block their target they moving from one side to another. This is something you MUST learn to become a good dart player.