You can always get personalized gift certificates; it's perfectly acceptable these days. It also makes it much easier on you to find the right best man gifts. But the thought behind the gift is the most important.

It wouldn't be tasteful to get cuff links (the standard best man gifts when you don't know what else to get your best man) when the only time he would ever wear them is at the wedding. Try to choose something that will leave an impression you thought about his needs and not just picked the first thing in the store. Also, try to remember his best man speech will come after you give him his gift.

The best gift you can give your best man is the one that is associated with a memorable event that he will treasure just as you will treasure his best man speech.