When painting your interior walls, to get the best finish you will need to apply more than one coat of paint, it is best to do at least two, sometimes three or four. If you try to cover in only one coat you will get a patchy finish. The paint should not be applied too thickly and a good quality medium nap roller should be used to work the paint across the wall.
Well you know how a bad workman blames his tools? When using gloss paint it may well be your tools letting you down, a good quality brush is essential, a black china bristle brush is the best tool for the job! Don't load your brush with too much paint as that is sure to lead to drips in your gloss. Using long soft strokes going back and forth over the same area will help to ensure an even thickness preventing those annoying paint drips and runs! Don't be scared to thin the paint a little, although it may seem strange the thicker the paint the more likely it is to run.
When not using a professional decorator many people don't properly prepare their surfaces before painting. Preparation is key in getting a good finish, all surfaces should be rubbed down with sandpaper first to give a smooth surface with some key to it. Once rubbing down is complete it is really important to remove the dust that has been created on the surface, the best way of doing this is to use a "tack" cloth, however a normal damp cloth will do the job but you will need to allow the surface to dry before applying any paint.