Which version of LSWS are you using?
Is the memory usage of lshttpd process keep growing or that of php processes?
Can you please post your "top" output sorted by memory usage.
Usually, lshttpd only use about 30MB at most, if the memory usage of lshttpd grows beyond that, it is likely caused by memory leaks.
We just fixed a possible memory leack in 2.0RC3 when .htaccess is used. Is "Allow Override" enabled on your site?
If you are interested, you can try our intrim 2.0RC4 release, which is very stable already.