You need to create a new device if your current device is all used up.
To create a new device, first find space. It depends whether you are using raw or filesystem.
Once you find space, you create the device using 'disk init' and to extend the database you use 'alter database'.
Please read up
http://sybooks.sybase.com on these commands.
Just checked the heading and Im a bit confused. If you are looking to do automated log truncation in checkpoint, you need to set-up the db option 'trunc log on chkpt'.
sp_dboption 'dbname','trunc log on chkpt',true
go