I'm a tad bored, so I decided to write up some notes on bannermaking. I hope this is helpful to both the people who don't know how to make banners, and the people who make awesome banners <3
 Match the icon as best you can. This may be hard for some icons, depending on the overall look of the icon, the amount of text you need to add to the icon, and the textures/brushes available to you. However, in my opinion, the banner should echo the elements in the icon. Color, texture, brushes, even the text could match up. If there's a swishy font in the icon, use a swishy font for the banner. If there are shades of purple in the icon, use shades of purple for the banner. If there are tiny text brushes in the banner, play with tiny text in the banner. It takes creativity to make the icon and banner fit together, but after you get practice it gets easier to visualize how you want it to look.
[x] Make sure the text is readable. The worst banners are the ones that you can't read. If you can't read the text either change the color or add a drop shadow. It takes some work to get it right, but it's totally worth it in the end
[x] Font choice is really important. Especially if you don't have/know the font used in the icon. Usually the case for most people is “It's close enough!” and that's great, don't stray too much from the style of font. If it's a blocky font, use another blocky font if you don't have the same font. If you have the same font that's used on the icon you're working with, use it! If you think the font looks familiar, try to find it. I've spent long periods of time just trying to find a font that looks similar to the text on an icon, and it always pays off. Also, don't just pay attention to the type of font, but the way the text looks on the icon. Try to match the typography to the icon if you can! Little attention to details like that can make a good banner a super awesome banner! Font color is also super important.
[x] Don't be biased. If you don't like the icon you're working with, don't do a worse job. Remember that you're name is going next to the banner if the icon maker is crediting. Do you want your name associated with a bad piece of art? Matching the icon to accentuate it will make it a good banner, even if the icon itself isn't something you're a fan of. Also a great example for things that are hard to banner for. I personally have trouble picking textures to match heavy image icons, so this is often something that's a big challenge for me.
[x] If you've been provided with textures, don't let them get you lazy. Being provided with the textures people used is a great advantage... or so it seems. But it also can make you lazy when working with banners. I see a lot of banners where the texture was flipped twice on the banner and text slapped over it. Overall, the banner is fine. It matches the icon, and the colors match up. But for a great banner, we shouldn't be able to see the flip marks. Changing the size of the texture, erasing, and light blurring usually help the gap to fill in a bit more. Sometimes I don't even use the textures provided, instead I choose a similair texture and change things to make it match. This will often be better then trying to figure out how the icon maker used all the textures to get the effect that you're trying to duplicate.
[x] Attention to details is what makes the banner go over the top. It's as simple as adding a bit of tiny text, making sure the texture lines up to the one used on the icon, or adding something that looks like an element in the icon. These little things can really give a banner the extra oohmph
[x] Don't overdo it. Too many things going on can ruin the banner. Simpleness makes a clean banner. Although there are always acceptations, like a busy icon looks good with a busy banner, or the textures you use to match the banner need a little love. At the same time, don't underdo it. Even in the icon is super simple, don't just copy the color. I usually like to add a bit of lighter or darker color so that the banner section pops and adds a little more interest to the overall look.
Any questions, just ask! I may make a bannermaking tutorial too, if anyone is interested.