Many authors angst over their pseudonym before they even finish a manuscript. There are a few reasons to think about it at this point in your career.
1. Name recognition--if you are active writing or commenting on bl0gs, yahoo loops, twitter, etc. then you want people to get to know the 'new you'.
2. Domain--buy it before you become famous and having to deal with some idiot holding it hostage.
3. Multiple genres--if you write in drastically different genres, it would be wise to keep the two separate. For example, I've written numerous children's books (PB's or picture books), romantic suspense with paranormal elements, and dark urban fantasy. The PB's will definitely be under a different name! And hopefully, will never be mixed with the other two!
4. Sexual genres--goes along the lines of #3. For example if you write erotica, live in a small town married to the local pastor, or teach at the local elementary school, then it would behoove you to use a pseudonym.
5. Use a short name--remember all those author signings you want to do? You have to write your name A LOT! Make it easy on yourself.
6. Use a name you will answer to--people will be calling you by your 'new name' be certain you will actually answer to it! Don't go totally weird and different unless you already answer to the totally weird and different name.
That's all the advice I can think of for now. If you have any additional ideas or suggestions, please respond in the comments.
Write on! Time to exercise on the Wii Fit before writing my daily word count!