Tips

01) Know the basic tools and functions of the software you're using. 
In this way, you'll know you're way around Photoshop/Gimp/etc. and
you just can't start editing unless you don't know anything of your editor.

If you don't know the basic tools of your editor, then check this out:
click for photoshop || click for gimp.   

02) Simplicity is beauty. Do not overcrowd your poster, it 
would simply look messy. Although there are some designs who managed
to look great even if it's crowded. 

03) Use the proper resources for your poster. Use the right textures, stocks
etc. that would fit your poster to avoid confusing genre. 

04) Use high quality pictures! This is very important. 
Using low quality pictures would make the poster sore 
to the eye. Also, I would advise you NOT to use fan-taken pictures.
Photographers watermarked them for a reason. 

05) Make sure that the character's expression on the poster reflects the
attitude of the character on the story. Do not put a smiling picture 
of Myungsoo when all he's  a cold jerk on the story (he typically is). 

06) I probably said it but use the right stock for the poster. Do
not use a piano when it is not mentioned in the story. Do not
use a castle when the setting is in the city. 

07) Use colors that fit well together. You may experiment with colors
but make sure that they compliment each other well. 

08) There's no such thing as designer's block. There's something in
your brain that needs to get out, then get it out.

09)  Be original. Do not copy other people's work, if you do, 
you'll only lose your credibility of being a good designer. Use your imagination
and design your poster as your imagination seemed to fit. 

10) This is a bad tip: recreate a graphic. If you don't like the graphic,
then point out the negative things about it and  redesign it
and create it your own. But do not make it seem like you're 
copying.