Get yourself one of those metal triangular rulers. You can usually find them at a craft store where the doll house making supplies are at. I use that and my heat tool to cut along the lines. I get perfect V cuts every time.
I bought mine on ebay...here is the link: X-Acto 3" Precision Stainless Steel Ruler Triangle 7725 - eBay (item 260346470741 end time May-13-09 12:39:08 PDT)