Some things to mask:
- Side windows
- Rear window
- Rubber gaskets that you weren't able to remove in step 2
- Door handles
- The inside of the car (you'll have the doors open when you paint)
- Tail pipe
- Engine bay
- Radiator (believe me, it looks quite retarded if paint gets on there when you paint the front of the car)
- Any important-looking labels inside the door that have important car information
For big areas use quality masking paper or cardboard, and garbage bags work well for tires.
The quality of the mask job is immeasurably important. If you do super crisp accurate masking, your paint job will look like the car was always that color. Spend as much time on this step as possible.