Measure, Measure, Measure
One of the main causes of a dry cake is using too much flour and this is usually done by using a 'heaped' spoon instead of a level one (unless the recipe alls for a heaped spoon) - it all adds up. Make sure you level off your spoon before adding to the mixture.
Additionally, adding too little liquid can also cause dryness - don't skimp on the oil if the recipe calls for it in a bid to make it 'healthier'. Instead, you can use alternatives such as apple sauce.