Corner placement will give you the most output, but not necessarily the best output. You might find that the side wall is better, it greatly depends on your room.
An alternative option to Bradleyfs suggestion:
If you have a laptop and a SPL meter, download Room EQ
http://www.hometheatershack.com/roomeq/ I haven't used it yet but it appears to be very easy reading the help files.
Place the sub at the main listening position. Now, using the software mentioned measure the frequency response with the SPL meter at various potential spots for the sub. Look at the data and decide where you get the best FR, place the sub there.
Once you have it in it's new position, level match it with the rest of your system. This is a very general guide but should get you started.