My pencil of choice is the Pentel Graph 1000 for Pro. I heard about it over at MegaTokyo ages ago.
I have been using my current Graph 1000 for around 3.5 years now, which is since the start of college. My previous one I used for 4 years throughout all of high school, but it got lost.
Unfortunately, this model is discontinued now and replaced with the Graph Gear 1000. I prefer the old version and I’ll probably pick up some spares when I find them.
But seriously, just shell out $20 and get yourself a mechanical pencil that can last you for a decade or longer.