Before putting paint on the wall, plan your painting and don't rush. As a quick guide, it can take six hours to do one coat in a 4x5m room, and two to three days to paint a room completely. Using a roller you can cover about two walls an hour, and it should take less than a day to cover the ceiling and walls.
Good technique involves always starting from the top and first doing the ceiling, cornices and edges where the wall meets the ceiling. Then move on to walls, doors, window frames and architraves. Start with the ceiling to allow for accidental running down the walls, as you will be painting over this later so it doesn't matter. It's also a good idea to wear goggles during this part for eye protection. Another good tip for painting the ceiling with a roller is to start away from the main window and work towards the light, to avoid painting in shadow.
Paint the walls starting at the edges and doing corners with a brush. When it comes to the trim, paint it using a brush, working along the skirting and then doing the door and window frames.
Try not to paint too large of an area at one time. Overlapping occurs when a freshly painted section begins to dry before you start painting the adjoining area, and it's best to finish one wall at a time to prevent drying and streaking.
When taking a break, don't forget to cover brushes and rollers in plastic wrap or clean them completely. Once the paint is distributed and the roller almost dry, a handy trick is to go back over the whole area in long even strokes to create an even finish.
When painting a big job outside, wash your brush out periodically to prevent paint build up. It can also help to have two brushes, with one kept sitting in water and then swap over when necessary.
It's important to apply any touch-up by the same method as the original application if possible, to avoid having any difference in sheen or texture. Be sure to use paint from the original batch, reduced 25 to 50 percent, and only apply a thin coat. It's also advisable to apply the touch-up paint under similar temperature and humidity conditions as the original.