No matter how well you can draw; how readily you're able to translate your ideas into
3D, there will be times when those ideas don't come so easily. Sometimes, it can be downright difficult. You can hit mental roadblocks or you may feel that you can't sketch out your ideas because of limited skill with a pencil or pen. Whatever the reason, there will be times when you'll need to look to some different avenues to jump-start your idea generation process.
Using Reference
When coming up with concepts for your 3D characters, props, or environments, it's vital to get good reference. Reference can come from elements of your particular location or you can jump onto the internet and find images from anywhere in the world. You can also create scenes and scenarios and photograph those yourself. Think about a few elements of your design that can be grounded in something real and build on those, combining those disparate elements in interesting ways.
For instance, you could combine the very common sight of someone leaning against a wall, face buried in their phone, with the features of a baby pig. Just comb through lots of images and many times a few things will stick out that fit your idea or strike you as interesting.
Thumbnails

Even if you feel like you can't draw, don't be afraid of thumbnails. They can be as simple as silhouettes and they're meant to be created very quickly. Don't worry about details, just get down as many ideas as you can.
Once you've done that, take a look at the silhouettes and see if anything strikes you, then work up some slightly more detailed drawings for those shapes or even combine some of your favorite elements into a new sketch. Working rapidly on overall shapes and thumbnails can provide a great jumping off point in moving forward with your design.
Sketching

If you're blessed with the ability to draw, then this is obviously a great option for quickly working up ideas. But, even if you're not as confident in your skills, try putting down some more fleshed out sketches on paper (or digitally). No one else has to see this if you don't want them too.
These are really to help you get some ideas together before creating your model. In the end, if it helps you accomplish your goal then it was a" good" drawing. And as a bonus, the more you practice, the better you'll get!
Photo Manipulation

Another method for creating custom reference is to combine existing image elements into a new concept image. So, instead of stopping at collecting and looking at reference images to give us inspiration, you can use the images themselves. For instance, when creating a cartoon-style police car you can take photographs of actual cars and begin to cut the images apart in a program like Photoshop.
You can then distort the different pieces and combine them back together to create a reference that gives you a really good idea of the direction you may want to go. It doesn't have to be a finished image, you're just using it as a guide for how things could look and fit together.
3D Concepting

Finally, you can skip the sketching and photo manipulation all together and go straight to 3D concepts. A program like ZBrush makes this possible because you can quickly create and modify overall forms. You can add or remove limbs, move shapes around and generally exercise a lot of freedom in working up different concepts.
This is a popular way of working and has many advantages. It's great for people who can better visualize their work in 3D and you can often use the concept itself as a base for a final production model.
No matter how you use reference and concepts, it's an important step in creating any 3D character, prop or environment. So try out a few methods and see what works best for you! If you want to learn more check out this great tutorial that teaches you how to go
from concept to a completed 3D model.