Your most valuable sex organs are your eyes, and your ears.  Of course, that other organ located between those ears is also pretty important, so you can know what to do with what you see and hear.

Eyes

Even in the flirting stage, your eyes are one of your most valuable assets in being a good lover.  You can tell by your partner's (or potential partner's) a great deal about how the rest of the encounter is going to go.  It's called body language, and people say much more with in this language than they usually realize.  Women will often lean closer to you, or maybe even touch you on the arm, shoulder, chest, etc. when they desire to get physical.  Men will often appear to snap to attention when you say something that is particularly arousing to them. 

Once it gets to the physical state, your eyes become even more valuable.  By observing your partner, before, and during sex, especially during foreplay, you can tell how it's going every step of the way. By watching the reactions of their body during kissing, touching, groping, even oral sex, you can easily figure out the things that are working, and the things that are not. If their body shudders when you touch or kiss a certain spot, that's a good thing.  If you get no reaction, or a defensive flinch, you're probably not doing something right.  So, keep your eyes open, and pay attention to what's working, and what isn't.

Ears

Coupled with your eyes, your ears can also help you a great deal in being a good (maybe even great) lover.  Again, in the flirting stage, being able to hear the way things are said is even more important than hearing what is being said.  A person's tone can sometimes completely betray their true intentions.  For instance, a woman can be saying the dirtiest, sexiest thing imaginable, but if she's using that Ben Stein-type monotone ("Bueller....Bueller"), it kind of takes the sexiness away from it.  Likewise, a man can be saying something totally innocent, but his tone can indicate that his intentions are far from innocent.  

Once things get physical, your ears also come in handy a great deal here too.  If you try, you can hear someone's reactions as good as you can see them (sometimes even better).  You may hear a short gasp of breath, or a soft moan if you are doing something right .  If you hear nothing, that's usually a good indication that a different approach may be necessary.

The Brain

Finally, you have to use your brain to put together all the things that are working, and the things that aren't.  Just because one thing you do is arousing your partner, it may not be a good idea to just keep doing that and only that.  You have to pay attention to all the things that your partner was enjoying, and alternate between them, creating an intense experience that neither of you will soon forget.  Of course, one exception to this is that, if a person asks you to go back to doing something that you have stopped doing, then by all means, please listen to them, because they know better than anyone what feels good to them.

So, as gifted as you may or may not be in various physical endowments, your eyes, ears, and brain are your most valuable sex organs.  While being an Adonis or an Aphrodite may give you all the physical tools to be a great lover, it's not a necessity, and even if you are blessed that way, you need to use your other sex organs to make the experience as pleasurable as possible for both of you.