Sometimes I see someone with a good sense of design, but ambition got ahead of their skills at the time. Biggest example is drawing the human figure without knowing how or finding ways around it - this one happens everywhere. It's disappointing to see a nice image shout amateur because one element gave the artist trouble, when there are a ton of resources out there to help.
In other words, stop and reassess what will be in the final design if a part of it is creating a lot of trouble.
"Use what works. Ditch what doesn't." - ansharp
Reminder - this applies to designs meant to be sold, and need a polished, professional look. Pushing yourself in practice is good!
Ways to avoid this problem:
a) Find references to learn from. Don't know how to draw a cat's leg? Look up images of cats to study, or even a skeletal/muscular illustration for an in-depth approach.
What to watch out for: Referencing doesn't mean directly copying or tracing - carefully avoid it when looking at another person's photograph or illustration. Eyeballing can still result in a copy!
b) Find stock images. Sometimes people offer their photographs - of themselves and things around them - and allow commercial usage with no fee. There are also pay sites to consider.
What to watch out for: ALWAYS read usage guidelines carefully. While some stock sites allow commercial use, not all of them do and others have special requirements.
c) Take photographs for reference and build up a collection. A large, varied collection is a valuable tool - not to mention no conditions to worry about.
d) Ask a friend to pose, or use a mirror to pose something you have in mind.
---Hopefully, I will have part two up next Thursday. :)