Unless your stripes will have varying or random widths, you'll need to plan ahead by measuring the walls. Calculate the circumference of the room in inches and divide that by the width of your stripes. Most likely, the measurement won't be exact, so paint the off-width stripe in a corner where it won't be noticeable.
There are several ways to lay out straight lines for your stripes. The easiest is with a laser leveler, which shines a perfectly straight line of light on the wall, allowing you to precisely apply tape. A reliable, old-fashioned chalk line works well. Mark the location of each stripe, have an accomplice hold one end of the chalk line and snap a straight line along the wall. Apply painter's tape along the lines, pressing it down firmly to prevent bleed through.
The more contrast between the stripes and the deeper or brighter the colors, the more dramatic the effect. Narrow stripes in bright or highly contrasting colors can look busy when applied to all the walls in a room and may be more effective as a focal wall. For a subtle look, use the same color but two different sheens of paint, such as pale beige with alternating stripes in flat and semigloss paint.
There are several specialty paints for adding texture and depth to stripes. These include metallic paint, suede-look paint, sand paint and pearl finish paint. Alternated with flat or satin wall paint stripes, specialty paints can look dramatic. Often, these are best used in smaller rooms, such as a guest bath, or for the walls above wainscoting or a chair rail. Like bright or highly contrasting colors, some specialty paints may look a bit overwhelming when an entire room is striped.
There's no rule that stripes are for walls only. A bank of plain closet doors in a bedroom isn't very attracive, and continuing the stripes from the wall across the doors minimizes their appearance.