because that's its what i tend to do only in my life drawing classes. Maybe its from how far or close i stand?
anyway i can eventually find the "right" proportions but my stuff gets too sketchy and messy.
But that issue isn't a problem when im not drawing in the classroom
the paper is kinda too big to scan
maybe ill just make an example on my paper
Not really. You're most likely are not gonna get a line 100% accurate the first time. Drawing is a process of continuous revision.
Its not the lines you make that are the problem it your intent when you make them. Instead of scribbling, think about each mark before you make it, decide how you want it to look. Every line has a beginning point, a path it travels and an ending point. Know what those are for each line you make, before you make it. If it isn't right, correct it with another line that you also decide on before you make it.
"Guessing lines" are inevitable with most but I have found a direct correlation in students between developing a strong, confident line and growth of the ability to see. I have all my guys draw from life with a single, if possible another line or two won't hurt, considered line in their sketchbooks for me. These are exercises and should be smart and considered. There are many others ways to work in a sketchbook but for me, and these specific exercises, do it my way. The growth in most has been measurable.
When you draw trying not to make a mistake you are weak young padawan, when you draw from confidence (there is no wrong) with consideration you become strong.
I just demonstrated this exact thing to my concept art students...but in a way opposite of what Bill just described (though this is drawing and construction from imagination rather than observation). I notice that students unfamiliar with the process of drawing and mark-making put pencil to paper and try to draw an ellipse with one solid line in one motion...inevitably it is off, wobbly and kinda painful to look at. I tried to demonstrate to them that you can draw a very accurate ellipse, sphere, whatever by using what bill618 described as "ghosting gesture"...you make the mark or line's motion loosely before even touching the paper...lightly laying in many strokes (but still with confidence) and building the shape until it feels accurate. Then going in and defining stronger, sensitive lines that define the form...the "searching/ghost" lines can be ersaed, left to add energy, built up into value, etc.
(BTW...not at all disagreeing with Bill...this is a significantly different application and goal)
Check out Syd Mead's work if you want to see "searching lines" work for you.
And after a quick re-read of the OP I see Pete was asking specifically about figure work from life...so maybe my response doesn't apply that much. Personally I still work pretty much that way though, even from life...building on the gesture and roughing in basic form...then defining and accenting as needed.
Aside from the excellent advice given above, I just wanted to say that I learned a good technique for learning better line accuracy in one of the Gnomon tutorial videos, which goes along well with the "air drawing" (like air guitar, right?) techniques above. As well as observing where your line should go and doing practice passes first, try putting two dots on your paper a fair distance apart from one another, and then try to join them accurately with a single smooth curved line with your pencil. It's tricky at first, but as you get better at it you'll find you are able to do it with the dots further apart and with different and more complex curves. You can do this lots of times over and over on the same sheet of paper. It's a bit like a pianist practicing scales, and with the same object - learning good motor control of your drawing hand.
Being able to draw with single confident strokes is one of the things which really distinguishes a skilled artist from an unpracticed one, but the good news is it's just a matter of practicing it!