Good drawing exercises...?

I would say just to start simple and move up. Maybe instead of drawing a whole portrait, for example, just draw eyes, or mouths, or hair ect... Of just practice more. Its kinda the same with everything, if you don't do it for a long time you lose skill. I used to do gymnastics before I started drawing, and I was an amazing little gymnast. There was talk about olympic training, but I didn't want the competition so I quit to find my real passion. No it's been 7 years and I can't do it like I used to. I've forgotten pretty much everything.. It's probably the same with drawing, you just kind of have to re-teach yourself how unil it comes back to you.

I dont think tracing will be of any help. Tracing teaches you nothing.

You should draw free hand to make yourself learn to see and put it on paper. If you feel difficulty drawing free hand, then first use grid method. When you get used to using grid method comfortably start reducing the no. of lines gradually untill you reach to just one horizontal and one vertical line. When you feel confident enough, switch to drawing free hand.

The best thing you can do for a drawing exercise is to trace. If you draw freehand, you make mistakes. Practicing drawing freehand means you're just practicing making the same mistakes. In martial arts, they teach you the exact moves, make you practice them exactly until you can do it without thinking. When you practice tracing, you practice moving your hand to the exact positions you need to trace the image. Oonce you get to the point where you can trace what you see in front of you without thinking, you'll then be able to trace what you see in your mind.