Wipe your walls down with sugar soap before you start. You can buy ready mixed stuff, which is easier. Using a clean cloth or sponge, start at top and work down. Make sure you rinse the cloth regularly, and also wipe the wall with clean wet cloth after to remove residue.
The sugar soap will remove dirt and grease, and give you a good base for your paint so it holds well.
Crown paint is better than dulux. Homebase have been doing 3 for 2, which is a good deal. (much cheaper than at B&Q)
Also, as well as using masking tape on skirting and light fittings etc, you can use it down the corner of a room to get a good clean line if you plan to do a feature wall of a different colour. Mask the one you arent painting first, paint right up to (and slightly over the tape). Carefully peel back the tape once the paint is dry.
Then, once you are sure the wall is 100% dry (i usually leave it a day or 2), then carefully tape along the wall you have painted. Once you have painted that wall, peel off, and you should have a nice neat line.