Looking for a good way to hone your skills?Often various CG forums will offer monthly/weekly challenges/charrettes. These are excellent ways to improve your CG skills. Generally these monthly/weekly challenges do not offer official prizes or even designate anyone as a "winner". Which is great because it makes for a very friendly and inviting environment. Now even though you probably won't receive a prize for your efforts you will still benefit from the feedback/advice that others provide on your work.
By participating you'll often pick up a new tip/trick that allows you to render more efficiently or makes your work more realistic, etc.. Therefore my tip today is to go out and participate in these challenges/charrettes regardless of your current skill set. BTW, if your fairly new to 3d don't have the mindset that your work isn't up to par so you can't join...jump in! At the end, I bet you'll be glad you did.
As you're probably aware there are many CG forums scattered about the internets. Here's a couple to get you going.
If Architectural Visualization is your bag, then check out http://www.vizdepot.com/ and http://www.3dallusions.com/forums/design-challanges-charrette-s-themed-challenges/. They currently have some nice charrettes underway right now, and you can still participate in older ones.
There's also some great lighting challenges right here at CGSociety:
That should be enough to keep you busy for a while. Join in, learn, help others, and have fun.