Just like a physical hard drive, a virtual hard drive can also be prone to slowdowns after months of usage if care is neglected.
After fixing up a few computers for friends who are not exactly tech-savvy, the speed improvements after a disk defrag is quite surprising. Normally they expect a computer to be formatted after a year of usage.
Use VMware's Disk Defrag:
To help maintain optimal speeds with the virtual drive, a disk defragmentation tool is bundled with VMware.
You can find it in the "Edit Virtual Machine settings" link under the "Hard Disk" item. Click on "Utilities" and then "Defragment".
Shrink your drive:
This will reduce the bloat in your virtual machine, removing unnecessary data such as snapshots. Depending on the usage, I've found it to remove up to 6.5gb on my host computer after cleaning a 20gb virtual drive.
To access this, it requires "VM Tools" to be installed on the guest machine. Run the virtual machine and click on the VM Tools icon in the taskbar.
Go to "Shrink" and select the supported partitions you wish to shrink. Click "Prepare to shrink" and give it some time to work its magic. Once its ready, confirm that you wish to shrink and it'll do some crunching.*edit 22/02/2010*
It is now much easier to shrink your drives. Just go to edit your VM machine settings, select the HDD and select "Compact" from the "Utilities" dropdown.
There are more uncommon space saving tips here.