Hmmm... as an arachnophobic myself, I can vouch for the effectiveness of the tactic described by Ads, above. It certainly gives me the jitters, I can tell you.
However, I'm a good sport, and I don't take offense at such things, once I realise the nature of the "spider". Actual spiders are a different matter entirely, however.
In fact, I'm such a good sport that I'll add another useful tactic to this node.
Apart from my arachnophobia, I also have a mortal dislike for cockroaches. I used to live in the tropics, and I've had my fill of creepy things. One tactic that still works on me, though I now live in roach-free environs, is this:
Approach your victim nonchalantly, then carefully and very softly (simulating the pitter-patter of little feet, so to speak) place your fingertips on his shoulder, while saying, in a level and unexcited tone of voice: "Cockroach."
It really is amazing how effective the power of suggestion is - certainly, this prank never fails to elicit a startled response, and often a satisfying "Eek!".