This question already has an answer here:
How can I empty (clear) the logcat buffer in Android?
I use adb logcat from command line and pipe the output to a file, since the DDMS has a very limited buffer. At the moment, when I restart my app (after fixing bugs etc) the logcat buffer has data from the previous launch as well. Even uninstalling the app does not clear the buffer. The only way I've found so far to clear the buffer, is reboot. This is effective, but would like to know if there's an easier way.
Logcat options are documented here: http://developer.android.com/tools/help/logcat.html
The following command will clear only non-rooted buffers (main, system ..etc).
If you want to clear all the buffers (like radio, kernel..etc), Please use the following commands
or
Use the following commands to know the list of buffers device supports
For anyone coming to this question wondering how to do this in Eclipse, You can remove the displayed text from the logCat using the button provided (often has a red X on the icon)
I give my solution for Mac:
If at any time you want to clear the log, type ./adb logcat -c