A photographer friend has agreed to take some sexy pictures of me. I trust him, but I don't want them to somehow wind up all over the internet.
How do we transfer rights of the photos to me?
The photographer retains the copyright unless he or she specifically assigns them it to someone else. However, usage is a different matter. The photographer may not use an image of you -- where you are readily recognizable -- without your release. This is called a "model release."
If you did not sign a model release, the best way to proceed might be to tell him that you respect his copyright but don't give him permission to publish the images on the 'net. That is probably as good as you can get without getting into a lose-lose battle over usage of an image depicting you.
I should note that there are instances where someone did sign a model release and then got cold feet once they saw the images and asked not to allow it to be used. Note I said asked. Again, your best bet is to appeal to the photographer's sense of respect for you, the model.
I'm not an attorney, but the outcome of these cases is, as I understand it, far from certain. However, most photographers will understand that you want them to use discretion and will refrain from posting where you don't want them seen. Especially if reminded that you didn't give your permission.
Avoid this situation by not letting him take away the memory card in the first place. Have him take the pictures using your memory card and collect it from him immediately after the pictures are taken. This isn't completely foolproof as there are ways to subvert this (cameras which record to two media cards for example) but you trust the photographer. He ought to be amenable to this arrangement; if I were the photographer in this situation I wouldn't want the responsibility of keeping the pictures safe.
This also avoids any messy situation between you should the pictures get leaked. You are now the sole possessor - be careful!
A separate issue - but as others have said you likely need to sign a contract with the photographer assigning the rights of the pictures to you. This situation is known as work-for-hire. Consult your local copyright laws.
To transfer the copyright you need a contract that says you have the copyright - if such a contact does not exists the photographer holds the copyright and the only way to transfer it is by him giving you the rights (by making such a contract).
Owning the copyright gives you zero control of the situation if the pictures get out - all it takes is one web site in a country that does not respect copyright (Russia or China for example) and it's over.
Also, you trust your friend - but what do you do if the computer is stolen of hacked into and the thief publishes the pictures?
The only way to make sure the pictures are never used without your permission is for you to have the only copies of the picture (and even then you have to use appropriate security measures).
If you really completely trust your friend you don't have to worry, if he is trustworthy he will never release the pictures - on the other hand, if you have any doubt at all (and if you didn't you wouldn't be asking this question) your only option is not to leave copies of the pictures with your friend.
Disclaimer: I am not a lawyer, law differ in different countries, etc.
The Only Way
The only way to be completely certain is for him to give you the memory card from his camera after he has shot the images.
This sounds draconian, and he might not go for it, but it is the only way for you to have control.
On a practical note: go and buy a memory card (check what type his camera uses) and take it with you to the shoot for him to use.
There's one easy answer: pay him.
Put together a simple contract stating that the photographer will take photographs and hand over negatives and copyright in exchange for a reward.
It's very clear cut at that point and would be legally enforceable in almost any country.
Professionally speaking, as I've worked with a lot of 'newbie' models doing 'sexy' photos I've found that the mood of all but the most professional of models is strongly reflected in the end result. A model who is worrying about how the photographer is going to use the photos is not going to be fully commited to producing an outstanding result.
Worry lines are not sexy so if you're worried about having 'sexy' pics 'out there' then you're probably not ready to have them taken.
P.S. When asking or answering a legal question you should specify which country it applies to. Because model releases function quite differently in the UK compared to the US.
P.P.S. Just found a link to this very useful article (as it relates to US law) on this site: http://www.danheller.com/model-release-copyrights.html