I'm assuming this will be on the street (as opposed to be sitting in front of a trade booth or inside mall display)? If this is something meant for the street remember that you'll get maybe half a second of attention from passing drivers (between sips of lattes and texting) so you'll want something that a) is simple and eye-catching (single word or easily identifiable logo) and b) is easy to read. The examples you gave are all printed sideways which means it would be tough to read or recognize text at 30 mph. Keep in mind that anything outdoors is also at the mercy of the elements - it looks like these things are built to bend in the wind so you'll want to keep that in mind - how does your artwork look tilted at 90 degrees?
If it's something for a trade booth or inside you probably have a little more time to grab users - maybe 3 whole seconds. I'd still aim for something simple - a sleek logo or one or two words that will make people stop and possibly enter the store / booth to learn more.
Standard stuff like an easily readable font (the third flag is a little tough to read and I don't know how easy it would be to read it while cruising down the road or walking down a mall thoroughfare) and good contrast (white text on a dark color background or a dark color on a light background) are always a good plan. These look like they're meant to be read from one side only - I would make sure that, if the fabric is at all translucent, your logo or text is readable in reverse (the human brain is pretty good at picking out standard words in reverse).
If the flag you're printing is taller than it is wider (as these are) you may want to consider running your text vertically instead of turning it sideways like the examples do. That way, people won't have to resist the urge to turn their head 90 degrees to read your text or company name.