For me it's pretty easy to hold off orgasm. I think I got this way through self-sexual-exploration, starting at a very early age. I would practice holding out as long as you can while masturbating... see if you can go an hour or longer. Wearing a condom will lessen sensation and probably allow you to last much longer, there are also some kinds of desensitizing creams you can buy, though I've never tried the latter. It helps me to focus on what my partner is feeling and doing. Keep your mind on making your wife happy, and don't focus on what it feels like to be moving your dick in and out of her. Don't focus on how you feel at all, keep your mind on her. If you feel as though you are about to come, slow down, change the rythm, or pull out for a minute and switch to oral if you need a chance to cool down. If you are really close and need to do something immediately to avoid climaxing, try taking two fingers and pressing them firmly into your taint- the patch of flesh between your asshole and your scrotum. Inhale while doing this, and this may stop you from climaxing even if you are right on the cusp. I've also found that if I have air blowing on my ass (from a fan or open window) it's next to impossible for me to come, though I don't know if that would work for other guys or if it's just me. You may also want to practice more foreplay with your wife, or ask her what you can do to stimulate her better, so that she is more ready to climax herself by the time you are ready to move to full insertion.
