Hi, I am a second year design student, but I still have trouble making satisfactory sketches. I understand that it is important to always practice drawing, but I am having a lot of trouble deciding what to draw whenever I try to practice. I end up drawings cubes and ellipses all the time. Can anyone give me some advise as to what to draw when practicing?
I think you should start drawing designed forms you care about instead of boxes and ellipses. These fundamental exercises are good hand training, but design drawing is more about communicating visual ideas, and thats where the fun is. So do shoes, cars, backpacks, chairs, whatever, but start “designing”, and do it over and over again. You’ll feel you have accomplished something.
Something that helped me (and still does) is to Sketch things you are interested in, it will keep you motivated. And try sketching them using different methods.
For example, if you love cell phones. Try getting a stack of cheap paper and just attacking it thinking of phones as you go. Try not to design the object in your head and then try to draw it. Think on the page.
Try throwing a few lines down randomly and see where that leads you. Then try starting with a detail and building from that, then maybe just sketch general forms, then just keypads. Or just back sides of the phones, try just sketching different ways to open the phone (besides hinges and slides) Sketch out 20-30 pages, pick the best elements from each page and try to draw 3-5 designs that synthesize the best elements of you exploration… still the way I work most of the time.
stay loose, fast, and keep a smile on your face. 
For training I practice the straight lines but I’m good at those but what I’ve found that really targets your accuracy are pages of concentric ellipses. Doing fast circles inside of each other that are pretty clean/round (no flat tires or eggs) and all on the same center really trains your accuracy and control. My drawing skills have improved tremendously just doing those drills so that your hand will do what you want it to where you want it to. I’ll also do a series of 10 circles the same size right over each other, non-stop. You’ll find they go all over the place and you’ll end up with a thick fuzzy circle. The goal is to make that thickness smaller and smaller (more accurate). And no doing them with hard tipped tools so you can stay in the groove and cheat, light strokes!
A exercise i used whilst at uni
open a magazine, look around what ever, pick and object (only needs to be small, coffee cup, remote etc) look at it for 10sec or so, then sketch it completely from memory.
As you get better shorten the length of time observing the product and limit the time it takes to draw it.
Idea is to get faster
i agree, quick observation sketches are a good way to learn how to get the bare minimum “important” stuff on the page quickly, like when you are watching tv or chilling somewhere break out your sketchpad and try to record that image in your mind and put it onto the page, the goal is not quality rather learning how to see so you do not focus so much on the details but the overall impression of thing you are trying to sketch…