i would say the most important part of a sketch in our field is for it to be explicit. by that i mean that it should explain exactly what it does, looks like, scale, everything. this means that it should break away from total realism, in order to emphasize the parts that are the most important, functional, or the focus of the design. not to say that a sketch should be rigid and orthographic, but that it should explain to someone that is totally unfamiliar to the project exactly what is going on here. thats not easy, but that should be your main goal. you can make up for bad elipses in other areas. for the technical skills practice makes perfect. elipses are hard, that and controlling your line, but if you are constantly drawing, even stuff that you know if bad design,hehe, then in time the bugs will work themselves out.
for instruments, i do like a non photo blue, even though it shows up in scans : P. for that rough layer where i map out my sketch, as one would with a non photo, it gives a nice effect to use a light color that reflects the feeling of your concept, if its a cold concept, then non photo blue, if its a warm concept then perhaps a tan or light orange. also a nice light grey always works. at first i think that layering your sketches is the best way to make sure that you are in control. dont just jump to the sharpie, bc youll lay a line wrong and theres no turning back from there. start light and work dark, defining your form as you go.
thats about it for now , ill post more if i can think of what helped me to get better.