When exactly is a good idea to draw from memory versus drawing from life?
Im really good at drawing from life, even better than i am when drawing from memory. I tend to follow better than drawing out of my head. I want to know what is the best rule of thumb, (if any) like Vilppuu says "theres no rules, just tools" to drawing in a sketchbook from memory and drawing from life.
I have a hard time coming up with things in my sketchbook unless im looking at something. So how do i combine all the things i have learned, looked at, and practised in previous sketchbooks and generate new ideas?
"Have the courage to open your mouth, choose to voice your opinion, and express yourself."
I don't think there's any 'rules' around when to draw from life vs. drawing from memory.
Why should you ever draw from memory? It's a great way to really sharpen your subject knowledge. For example, if you just thoughtlessly draw from a live model but never really study human form, you are cutting your skills short. Drawing from your imagination forces you to really know your subject. You have to understand the gentle curve of the tibia, the way the pectoral muscle glides into the deltoid at the shoulder, the way the hamstring helps define the knee (and a 1000 other details) before you can really master figure drawing.
Drawing from your memory tests your knowledge and forces you to think of these details. Drawing from memory brings up hundreds of problems which only your imagination can solve. You won't be able to solve all these problems right away, but you will learn much by trying.
Copying from life already gives you all the answers--you just use your skill to see them and to put them down on paper. Cameras can do that so much better and faster.
Also consider the fact that sometimes what we see in life isn't always the most artistic way of representing your subject. Maybe the light has changed, or the shadows look odd, or the model moves, or the colors don't match. Being able to draw from memory allows you to fix numerous problems that arise during a live session. I'd encourage you to work from memory as much as you can, but use live models to make sure your work is accurate.
So, how do you start working from your imagination? Take it slow at first. Take a drawing you already have and draw it again from a different angle, or from a different lighting. Look at a live model, but don't draw what you see. Draw the model from a different perspective. Imagine what that model would look like from the floor or ceiling. Use a sitting model, but draw them standing. Use the model as reference, but don't draw it exactly. This will force you to look at a subject you are comfortable with, but in a different way.
Getting new ideas is a little different. Usually when I feel I've run out of new ideas, it's because I'm stuck in a routine. I'm listening to the same music, watching the same TV, talking to the same people, doing the same things everyday. I break out by changing things up--even a little change can lead you in new directions. Shake up your personal attitude a bit, open it up, and see what falls in.
Next, I'll look at what the great artists are doing (this website is a great place to do that). Study other's work. Find a style and try to copy it.
Switching to a different medium sometimes help get the imagination flowing.
Instead of drawing a thing, try drawing a topic, or a situation. Instead of drawing a guy with a gun, draw a holdup--a ruthless thug (give him a name, even) hell bent on getting the money and glory. Breath live into your pictures before you ever put down the first line. Fill it out with your imagination and let the details grow and feed on themselves.
Sometimes the ideas stop because of fear. We're afraid to draw something different or to leap into a new style because we may suck at it. It's easier to shut down our imagination than to try something new. To get past this, what I do anyway, is make my sketchbook my personal space. It's more secret than a diary. Nobody sees what's in there without my permission, and they only see what I want them to see. I've got pages and pages of crap in my sketchbook, but that's okay, nobody ever sees them unless I want them to.
Best of luck.