The Master Grid
Always draw a grid for multi-variable puzzles. Filling in 'X' for No and 'O' for Yes helps you visualize remaining possibilities instantly.
Start with the 'Fixed' Info
The biggest mistake is starting with a conditional statement. Find a statement that says 'A is on floor 3' and build around it.
If there are two possibilities, draw two diagrams. Don't try to keep both in your head.