An objection sometimes raised to performing oral sex is the taste. However silly it may seem that there is a product on the market designed to improve the taste of semen, it does fill an apparent need, and it does take the correct approach.
That's because what you eat determines, in part, the composition of various bodily excreta, sexual fluids included1. People who eat onions don't just have the smell on their breath, they exude it -- because it's secreted in sweat. Similarly, what you eat and drink will affect the taste of your semen.2
The first thing to remember is that everyone is different. The same diet may affect different people different ways, and some people may be naturally better-tasting than others. On the other side, different people will have different tastes -- there's pretty much no universal agreement on which foods taste good, and nor will there be universal agreement on which men taste good.3
That said, there are some guidelines that can help improve the flavor in most cases. Try to avoid the following:
Many of these foods tend to cause a bitter taste, while dairy products have high bacterial putrefaction levels and can cause semen to taste foul. In addition, avoid cigarettes, as smoking can also cause bitterness (aside from the fact that it's bad for you).
Foods that tend to improve the flavor include:
Acidic fruits and berries in particular can produce a pleasant, sugary flavor, while kiwifruit, celery5, pineapple, and watermelon can make semen taste lighter.
Now, like a number of people, I like practically the whole banned list. Fortunately, while permanent dietary changes may be more effective, according to Redbook, most of the forbidden foods will clear the system in about an hour and a half.
1Also blood, teleny points out, for those who expect to get bitten.
2I believe the equivalent is true for women.
3I've never gotten complaints, but that's with a very small sample size.
4Particularly Sunny Delight, digitalsulis reported.
5LeoDV says celery is said to make semen taste less acidic; machfive says it increases the quantity, if you're into that sort of thing.
Sources: http://www.askmen.com/love/dzimmer_100/117_love_answers.html
magazines.ivillage.com/redbook/sex/secrets/articles/0,12840,284446_289127-10,00.html