In Linux, you can't just "clean" memory like you would with disk space. However, you can control memory use by releasing cached memory, freeing buffers, or making the system drop cached data. Here are some commands and techniques to help manage memory:

1. Clear PageCache: To clear the page cache that holds saved file data, use the sync command and then echo 3 into the /proc/sys/vm/drop_caches file. This is usually safe for most systems, but it could cause files to load slower for a short time because the cache has to be rebuilt.

    sync && echo 3 > /proc/sys/vm/drop_caches
   

2. Clear Inode and Dentry Caches: To clear the inode and dentry caches (used for directory and metadata caching), you can echo 2 into the /proc/sys/vm/drop_caches file.

    echo 2 > /proc/sys/vm/drop_caches
   

3. Clear Swap Space: If you want to free up swap space, you can use the swapoff and swapon commands to deactivate and then reactivate your swap partition. This will effectively clear any data in the swap space.

    swapoff -a
    swapon -a
   

4. Unmap Shared Memory Segments: You can use the ipcs and ipcrm commands to identify and remove shared memory segments. This can help in situations where shared memory is using a significant amount of RAM.

    ipcs -m
    ipcrm -m <segment_id>
   

5. Restart Services and Applications: Sometimes, a service or application may be consuming excessive memory. Restarting it can free up the memory it was using. For example, to restart the Apache web server, you can use:

    sudo service apache2 restart
   

6. Increase Swap Space: If your system is frequently running out of memory, consider increasing your swap space or adding more physical RAM to your system.

Be careful when using these commands, especially on a production system, because they can affect how well the system works. Clearing caches might make file access slower for a short time as caches are rebuilt. Usually, Linux manages memory well, so you don't need to free up memory manually unless you have performance problems.