Try a different stimulation. Maybe switch hands or use both. I like to "edge" I find it very exciting to bring myself to the edge then stop for awhile then start again do this a bunch of times then let go. This bring on a very different feeling. Do you use porn when you masturbate? Maybe look at a different type of porn, change things up a bit see what comes up
That can and does happen to anyone, even during actual intercourse. It could be completely mental, such as not having the correct stimulus (try switching magazines :P), but it could also be caused by lubircant. Some, maybe most even, contain a retardent (lol) called 'benzocaine', which basically numbs your weiner so that you can last longer. Great if you're trying to impress some girl, stupid if your just whacking off.