Some work from the first week of CG Master Academy's Environment Sketching online course...
If anyone wants extra practice and is comfortable taking an online class - particularly for art - I would highly recommend CGMA. I have linked to their 2D academy page, but they've got a 3D school as well. Each has four semesters of registration and classes are about $700. Very much worth it, though. Intensive, informative lectures, you can choose what professor you wish to learn from (they usually give about 1-3 professor choices per type of class) and there's live Q&A, chats and student forums and everyone is there to learn.
Many students are extremely good; often they are already professionals who are looking for some extra practice, but it's great for the learning experience alone. At least for me, the students that are leagues ahead tend to get me motivated to do better and catch up, haha!
My friend and fellow artist, Terri Morgan, (who is one of the aforementioned amazingly good students) has taken a few courses from CGMA as well, and said they had helped her in her work. It was actually through her that I found out about them and subsequently decided to try a session.
Thumbnail sketches in 1-point, 2-point and 3-point perspective. (Another reason I decided to take this class; my perspective knowledge is lacking a bit.)
The first image is the rough sketches just scribbled in a graphed Moleskine - it's very thin paper and not really conducive to paint or even much marker, but I personally like the grid guidelines; they help me plan out aspect ratios and borders for such sketches.
The second is a cleaner draft from some favored thumbnails taken from the notebook, cleaned and rearranged in Paint Tool Sai.