Ask your gp what to do.  You'll probably be surprised with firstly, how common it is, and secondly all the different ways you can tackle the problem.  I'm sure your gp and you can come up with a great plan of attack.  Good luck!

One suggestion you could try (although your wife may not be comfortable with it) is for her to masturbate before you join in, that way she will be closer to orgasm when you go to have sex. You could also make sure you do this to her first and once she reaches orgasm then you can proceed to having sex. Also massages, kisses and anything to do with our necks is going to make your wife "get there" quicker. Other than that try thinking unsexy thoughts lol!

If it starts to ruin your relationship though perhaps you should suggest seeing a relationship counsellor or someone who can help out with your sex life. If you think your wife will be touchy about this just explain to her that you want to make her happy and you want to satisfy her just as much as she satisfies you and that you are willing to do anything to fix it. You will probably find that a professional has plenty more great tips. Also there are heaps of books on the market as well that you can read into.

Good luck! I hope you and your wife work things out!