Leah and I have an awesome time writing together. In fact, we're trying to get in sync to write something else together. When we're going strong, it's more fun than writing alone, because you get that sense of joy from your thoughts synchronizing and feeling like you're two halves of the same brain...
Like some of the other folks on your flist mentioned, Leah & I toss around story ideas on chat first. When we start writing seriously, we at least have a bare-bones plot, know what the major conflict is going to be, and how it's going to end. Then we each go off and write individual scenes in the story, and come back and share them, discuss them on chat, and refine our outline. By the time we've got around a third of the fic written, we usually have a list of scenes that need to be in the story to cover all the plot points, and what POV each scene is going to be in. Then we divide up the unwritten scenes, depending on who has a strong opinion about how any particular scene should go.
I should mention that we've been using either Google Docs (https://www.google.com/accounts/ServiceLogin?service=writely&passive=true&continue=http%3A%2F%2Fdocs.google.com%2F&followup=http%3A%2F%2Fdocs.google.com%2F<mpl=homepage&nui=1) and Zoho Writer (http://writer.zoho.com/jsp/home.jsp?serviceurl=%2Findex.do) to make collaboration easier, and so that we're always on the same version of the story. (they're about the same, btw, Zoho can deal with slightly longer wordcount than GoogleDocs, but for longer fics, you're going to have to break the document into chapters, or the program will lose part of the document.) Each system also has a chat module, so you can write and chat at the same time if that works for you (I'm not coordinated enough to do that too often.), and you can give each other access so that you can edit each other's work.
You have to decide how much you trust your co-writer with your work, and agree beforehand what is okay to do and what is not. Is it okay to edit something you wrote without pointing out the changes that were made, or do you have to go through a "track changes" or similar mode and let each other approve of the other person's changes? Do you trust the other person to tell you when to chuck something and rewrite a scene? Do you trust them when they say: "here, wait, I have an idea!" and *they* rewrite your scene? It depends, obviously, and the different amounts of trust and confidence in each other don't come all at once, or without effort. You can, and probably should, take it in stages.
I think the best place to look for a collaborator would be to talk to folks who have betaed your fic before, and who you have a certain level of trust in already, and whose opinion you respect. Especially those of your betas whose writing you like, or whose writing is simpatico with yours, so that it's easier for your styles to mesh. It's good if you can find someone whose strengths and weaknesses complement your own, so that together, you're better than either of you apart...
on teamwork...
Like some of the other folks on your flist mentioned, Leah & I toss around story ideas on chat first. When we start writing seriously, we at least have a bare-bones plot, know what the major conflict is going to be, and how it's going to end. Then we each go off and write individual scenes in the story, and come back and share them, discuss them on chat, and refine our outline. By the time we've got around a third of the fic written, we usually have a list of scenes that need to be in the story to cover all the plot points, and what POV each scene is going to be in. Then we divide up the unwritten scenes, depending on who has a strong opinion about how any particular scene should go.
I should mention that we've been using either Google Docs (https://www.google.com/accounts/ServiceLogin?service=writely&passive=true&continue=http%3A%2F%2Fdocs.google.com%2F&followup=http%3A%2F%2Fdocs.google.com%2F<mpl=homepage&nui=1) and Zoho Writer (http://writer.zoho.com/jsp/home.jsp?serviceurl=%2Findex.do) to make collaboration easier, and so that we're always on the same version of the story. (they're about the same, btw, Zoho can deal with slightly longer wordcount than GoogleDocs, but for longer fics, you're going to have to break the document into chapters, or the program will lose part of the document.) Each system also has a chat module, so you can write and chat at the same time if that works for you (I'm not coordinated enough to do that too often.), and you can give each other access so that you can edit each other's work.
You have to decide how much you trust your co-writer with your work, and agree beforehand what is okay to do and what is not. Is it okay to edit something you wrote without pointing out the changes that were made, or do you have to go through a "track changes" or similar mode and let each other approve of the other person's changes? Do you trust the other person to tell you when to chuck something and rewrite a scene? Do you trust them when they say: "here, wait, I have an idea!" and *they* rewrite your scene? It depends, obviously, and the different amounts of trust and confidence in each other don't come all at once, or without effort. You can, and probably should, take it in stages.
I think the best place to look for a collaborator would be to talk to folks who have betaed your fic before, and who you have a certain level of trust in already, and whose opinion you respect. Especially those of your betas whose writing you like, or whose writing is simpatico with yours, so that it's easier for your styles to mesh. It's good if you can find someone whose strengths and weaknesses complement your own, so that together, you're better than either of you apart...