One of the hardest things for artists is producing your best work at all times.
There is this idea that we should do lots of 'sketches', while this is a good idea it can also be a trap! The problem is that because it is a sketch and not necessarily a work that we will show to others or sell, we tend to take less care. We can rush and accept faults in the drawing or values.
Some principles you might like to consider!
Slow down. What's the hurry? Better to take ten days and produce something you are really happy with than waste five days only to produce work that is full of errors
Make every mark count and avoid mindless mark making. One way to achieve this is to regularly stand back to evaluate your last mark
With a handheld mirror, compare your reference and drawing. Its amazing the errors you will find that need corrected when you view your work in reverse.
Take a break. Most of us tend to spend too long at the easel in a session, as a result our concentration starts to fade and we stop seeing errors. After just fifteen minutes away from the easel, you will return with fresh eyes and you will see things you hadn't noticed in your last session
Try to work with real focus and aim to produce work that would allow you to stop at any point and be satisfied with the work.
Learn to look 'really hard'. Seeing is the real skill that all great artists have. Take your time to evaluate the shape, tone and value of the area you are trying to draw. This is linked to slowing down. Spend as much time as you can until you are sure you have captured the area in your mind. Then make a decisive mark! Step back and evaluate. Over time you will get faster and more accurate. At one time Virtuoso pianists could only play scales at a snails pace. In fact some still practice scales at a very modest tempo so that they can really focus on the sound they are making with each press of the key.
In the early stages, don't overcommit with your marks. Especially with graphite, light marks give you much more scope to correct later on.
Look for negative shapes. For example when drawing legs on a table, You might find it easier to focus on and refer to the shape of the space between the legs.
In the early stages of a drawing, restrict yourself to straight lines. As the drawing progresses and you are happy with the basic proportions, you can then start to refine the drawing to introduce curves and changes of direction. This is where lighter marks become important.
With oil paint, restrict the amount of medium you use to the bare minimum. And even then, use medium only if you really need to.
Draw from the shoulder.
Work with a restricted palette. This is really important for beginners. you will find your paintings will be more harmonious. A good palette to try is Titanium White, Cadmium Yellow, Cadmium Red (or something similar), Ultramarine Blue and Burnt Umber. You can produce a tremendous range of colours,
Try laying your paint on the canvas. By that I mean avoid smashing heavy, layers of paint into the canvas, particularly early on.
Deal with problem areas as soon as you spot them. This is particularly true when it comes to the drawing. A bad drawing beautifully painted will still be a bad painting.
Value management is possibly the most important skill to learn. And possibly the hardest to truly master. The human eye instantly spots when values are not quite right. A perfect drawing will not save you if the values are wrong.
Squint! Effective squinting will really help you to see values correctly.
Do master copies. As many as you can! With the internet we have incredible access to old masters. Sites like Google Arts list painting in very high resolution. You can get close to the work of your favourite artist to try to figure out how they worked. So many artists do not take advantage of these fantastic resources. I have met artists who refuse to do master studies because they say they have their own style and don't want to be influenced by others. I believe this is a mistake.
Don't be frightened to scrape down a painting or abandon a drawing. Even the greatest did this! You haven't lost anything. In fact you have gained, because you recognised that it wasn't' going in the direction you visualised. Having said that, sometimes you do need to persevere, even when you think it might not work. This builds resilience!