If you have the time (and CDP), one thing you may find helpful for TFE is to look at the answer section for one of the tests. You can (1) Look at the 3D object that they provide in the answer and try to draw the correct top view, front view, and side view. (2) Do the opposite. Look at all three views in the answers and draw the 3D image.
These techniques really helped me figure out how to visualize the object in three dimensions given the different views. From, there, just keep practicing. I know that everyone says it, but the more you do, the more you will get the hang of it. The post that Yankem suggested is also a great resource.
For the keyholes, don't fall for the traps. Sometimes, the answer will look just like the shape, but will have a small component that is wrong. Watch out for those. Visualize from front, top, and side and really just imagine that you are pushing the object through the aperture. Eliminate those that you know are wrong. For those you haven't eliminated, try to find commonalities (in outline, distinguishing characteristics and their relations to each other...) between the object and the keyhole you are examining. Don't be afraid of seemingly simple answers. Sometimes, the only answer that works is the rectangle or the square, even if the shape looks complicated. That's where visualizing the object from different angles comes important. But again, just practice these things.
Good luck!
