i call it a bubble lamp and not a lava lamp because in its off state the "lava" sits at the top of the bottle, but as it is turned on and heated the wax sinks according to the room temperature and remains a static bubble.
you will need:
+ power plug
+ glass bottle
+ metallic bottle holder (angle iron)
+ rubbing alcohol
+ paraffin wax
+ 30 watt globe
Step 1: Base & switch
you could go with the easier option of using a cable mounted switch. but i enjoy making mechanisms and creating more work for myself. so i went for creating a rotary switch using a brass knob from a drawer. i threaded a bolt into it and welded a washer off center to touch the switch, the video should explain.
invalid movie: http://www.vimeo.com/5487773
the base can be made out of whatever you want i used wood because it has a nice finish and made it much easier for me to router the shapes to create my switch.
Step 2: Bottle holder
when designing the bottle holder you should consider where and how you are going to place the light bulb holder. the light bulb should be about 5mm to right against the glass of the bottle, it needs to be close to conduct heat. if you plan on using a cylindrical bottle you may be lucky enough to find a section of steel pipe or something..