For best results paint during moderate temperatures and avoid painting when there is any risk of rain. Water and/or extreme heat can damage your freshly painted wall.
Always mix your paint until it has proper color consistency. Re-mix fairly regularly to ensure the color stays consistent. (If you are painting a large area you may want to mix several gallons of paint into a larger bucket to make sure you have consistent color over the entire surface.
Paint the top and work down leaving any trim work until the last.
When you need to take a break wrap up your roller or brushes in plastic to keep them from drying out. If you need to break for a longer period of time (overnight) wrap them in plastic and place them in a refrigerator.
If you are painting siding or some other surface that has a horizontal orientation work from side to side when using a brush to cut down on lap marks.
Use long smooth strokes when painting with a brush this will give you the most smooth and consistent paint application.
Make sure you have an extension pole when using a roller. Not only will this allow you to reach higher on a wall but will actually reduce your arm strain when properly used.
If you are painting a large house consider renting a sprayer which will speed up the painting process and may deliver a more consistent coverage. Paint sprayers can be rented inexpensively at many paint and/or rental stores.