BANNERS
Structural: Banners often are built in three layers: a front, a back and a center stabilizing fabric. It's important to use a center fabric that has stability in the weave. So a sturdy poplin, trigger or broadcloth is often a good choice. An old muslin sheet is even serviceable if your design fields are not too heavy. I've also used gabardine. The main thing is that it not be stretchy or loosely woven.
Draped fabrics: Since banners can have 3-dimensional elements, sometimes fabric draping is used in design work. It is a good idea to find some kind of knit to to this with as it doesn't crush and wrinkle easily. When one is constantly rolling up, packing and unpacking banners....non-crushable elements become quite desirable.
Backgrounds: I think that one of the errors many beginners make is to spend a big chunk of their budget on the background fabric. Sometimes, this is a good idea, an expensive particular fabric may be an important part of the message in the design....but in many cases, by the time you've put on all your designs and embellishments, very little of the background fabric is visibile anyway. So get something striking, but inexpensive, and put your dollars into the fabulous trims and lettering embellishments. It is good design to use related fabrics, or even the same fabric on front and back for continuity.
Range of Materials: Don't limit yourself just to fabrics. I've often used mylar, paper, netting, and other non-fabric materials to achieve the effects I want. I used quite a bit of irridescent cellophane in the flame designs on the Seven Spirits banners. It gave a great effect and easily was fused to the design with hot glue. Don't forget you have the option to be 3-dimensional. You can use beads, jewels, ropes, cords, silk greenery, pretty much anything that can be glued down. Scale is important...go for large pieces, or smaller ones arranged in groups that give the appearance of large pieces.
Rich & glitzy fabrics are what you want. Brocades, lames, sequin cloth, liquid lames, embellished velvets, silk, and anything glorious.
Fabrics to avoid: Satin, IMO, is a bad choice for banners. It glue-marks easily, and gets soiled looking quite quickly. Better to leave the satin for bridesmaids shoes. Flimsy fabrics with trivial designs. Avoid obvious florals unless it is part of the message. Remember you are garmenting the King. Don't put little poseys all over Him.
FLAGS
Flags, because they are used with more motion than banners, have different fabric requirements. Soft, fluid fabrics and simple shapes and symbols work best on flags. After all, as a flag whips by you, you do not have a lot of time to make out words or intricate images. Sometimes it is purely the color of the flag that carries it's ministry message...or even the shape.
Good flag fabrics: silk, micro-fibers like poly-silks, rip-stop nylon, lining fabric, acetate or polyester taffetas (great for getting that cracking noise!), and filmy translucent fabrics like organdies and irridescents.
Poor choices for flag fabric: anything heavy (like satin, brocades, denim, etc.) laces & netting(unless overlaid on another thin, fluid fabric--why? the air doesn't hold them up!), anything stiff.
Phlags & Physics Remember that anything heavy on the end of the flag is going to weigh it down and inhibit it's ablility to fly and move properly. Therefore, if designs are going to be appliqued onto the flag, the closer you put them to the pole end of the flag--the better. Although heavy fringes are a good idea on a banner to help pull wrinkles out....they are a bad idea on a flag because you won't be able to keep the flag in the air because of the weight. However, if you must have fringes on the ends of your flags, try mylar streamers instead. They are very lightweight and have the bonus of being spectacularly shiny.
It might seem like a good idea to use laces or net fabrics for flags because they are often very sheer and lightweight, however, fabric with holes in it will allow the air to pass through rather than "loft" the flag, so are not a good choice as the main body of the flag.
