For smoothest finish on interior walls with latex paints, try the Purdy Nylox brush. It's the softest bristle Purdy makes for latex paints, therefore it leaves the fewest brush marks, but it will wear quickly on rough surfaces or with the new alkyd paints. I agree with Workaholic that XIM's extender is superior to Floetrol.
For exterior surfaces I typically use about 4 ounces/gallon of latex and 6 ounces/gallon for DTM (direct to metal) acrylic latex paints for substrate temperatures above 75 degrees F or when I get caught in direct sunlight before I can finish an exterior surface. It provides a longer "open window time" for properly spreading the paint, which makes it easier to maintain a wet edge.
If you are using one of the new alkyd paints, check with your paint supplier for an extender specially made for this type of paint. A specialty paint supplier is highly recommended over the big box stores for this type paint and extender.
If the paint has been previously used and left open while painting or stored for a while, it may be necessary to add an extender to compensate for evaporation of some of the active components in the paint (resins + moisture).
Always pour the estimated amount of paint you will need into a separate "cut can" for painting and never pour any remaining paint back in its original container after the job or you will contaminate it with dust, debris, and dried paint from the substrate or the cut can. Transfer any excess paint to a clean container and label it with the room, the brand, and the formulation information in case you should require more in the future.