Dream's rules for good graphics

rule #1 - image quality

This is the absolute most important part of making graphics. Large, high quality images are the key to making good edits, even if it's just cropping an image down. You need to look for smooth, clean lines in the artwork. The easiest way to make sure you're getting good images is to use a gallery, My favorite for anime is animeseason.net, but I've done real life graphics as well, and those you can tend to find model specific galleries. The format an image is saved as affects the quality as well. NEVER use .jpg or .jpeg as these will seriously distort and destroy your hard work. I personally prefer .png for all of my graphics as it gives you the highest quality stats.
[example of a good image] - note how you can click for a larger version
[example of a bad image] - note the blocky nature of the edges (pixellation) and the small size

rule #2 - image matching

If you're wanting to blend 2 images together, you have to make sure the LOOK like they could go together. This can be a bit difficult with anime what with the different drawing styles of different artists, plus their color choices. As such, you want to try to find ones that look like they were created by the same artist. This is where the tag features of zerochan help out since you can search the specific character, and if it's been tagged, a specific artist as well. Otherwise, you just have to go by eye, which is a bit time consuming, but worth it in the end. That, or you could take the lazy way out and just blend the same image together.

rule # 3 - K.I.S.S.

Keep It Simple Simon. I've been making these for a few years, even been on a few graphics sites, and one thing I see way too much is people overdoing it with the brushes and textures. While it's good to experiment, there IS a line, and sadly, many people cross it.

rule # 4 - Be original

Like I mentioned before, it's good to experiment. Test out the different blending options, the layer options and the filter effects. The best way to learn what each tool does is to use it. Even if your project comes out looking horrible, you've at least learned what NOT to do with those tools. One thing that bugs me the most about graphics sites is that one person will find their own style and others will copy it like sheep. While artistry allows for creative licenses and inspiration from others, it just feels dishonorable to outright copy what someone else did and call it your own (call it a personal pet peeve, but that's just my honest opinion)

rule #5 - HAVE FUN

There's no point in doing anything unless you enjoy it. No one out there is perfect in anything you do, and there's always something new to learn every day. Programs, like anything else, require practice in order to understand how they work. Remember, an artist will ALWAYS find the flaws in their own work where others may not notice. As such, trust in what others say about your work. Everyone started out with projects they weren't proud of (I have quite a few myself)